This article was first published in June 2014; last revised in October 2018.
The ultimate goal in the fight against infectious diseases is their eradication. This goal has already been achieved for two diseases. Smallpox, a once very deadly human disease, and rinderpest, a disease that affected animals.
Diseases that could be eradicated next include polio, Guinea worm, yaws, Carrion’s disease, hookworm, lymphatic filariasis, measles, ovine rinderpest, pork tape worm, river blindness, rubella, syphilis.
In this entry we do not cover all potentially eradicable diseases, but focus on the most promising and most devastating:
- Smallpox and rinderpest, the two diseases already eradicated [jump to section]
- Polio, Guinea worm, and yaws, the three diseases that will hopefully be eradicated next [jump to section]
- Rabies, a disease for which regional elimination is under way, and which might be eradicated in the future [jump to section]
- Tuberculosis, HIV/AIDS, and Malaria: the three infectious diseases which currently cause most deaths worldwide. We explain why they cannot currently be eradicated and what is missing for them to become eradicable. [jump to section]
We also discuss some theoretical background of eradication.
- Which two criteria need to be fulfilled to make a disease eradicable? [jump to section]
- How can we fight the spread of infectious diseases? [jump to section]
- Why is it so beneficial to eradicate diseases? [jump to section]
- What features of a disease make its eradication easier? [jump to section]
The eradication of a disease is permanent and global, while the elimination of a disease is an achievement restricted to a specific geographic area.
- Eradication of a disease refers to a deliberate effort that leads to the permanent reduction to zero of the worldwide incidence of infection caused by a specific agent.1 Eradication means that intervention measures are no longer required, the agent, which previously caused the disease is no longer present.
- Elimination of a disease refers to the deliberate effort that leads to the reduction to zero of the incidence of infection caused by a specific agent in a defined geographic area. A disease can be eliminated from a specific region without being eradicated globally. Actions to prevent the disease from transmitting or re-emerging are still required once a disease is eliminated.2
The table below summarizes the key facts about the diseases we will discuss in this entry [Clicking on the table below will open it in higher-resolution]. Even though these are all infectious diseases, the variety of pathogens (the disease-causing organisms), available means against and treatments for the diseases is striking. Even the case-fatality rates — the likelihood with which an infection will lead to death — range from zero to one hundred percent.
The interactive chart below displays the annual number of cases of the diseases discussed in this entry: it includes data for the already eradicated disease smallpox and those diseases for which eradication is underway.3
The chart shows the global number of cases, but by clicking on "Change country" on the bottom left of the chart it is possible to see the number of cases in any country of the world.
It is not possible to clearly draw a line between eradicable and non-eradicable diseases. Diseases only became eradicable after scientific discoveries gave us the tools to fight them.
However two conditions are absolutely necessary for a disease to be eradicable and there are several characteristics of diseases which make it more likely for a disease to be eradicable.
1. It is an infectious disease
For a disease to be eradicable it needs to be a disease you can "catch", for example from other humans or animals, i.e. it has to be infectious. Non-infectious diseases, such as heart disease or cancer, cannot be eradicated.
2. Measures against the disease exist
To eradicate a disease we need to know of measures to fight its spread. The summary table above illustrates the diversity of such means against diseases.
Whilst the characteristics of a disease are biologically-determined or fixed, the available measures against the disease can progress through our scientific understanding and technological developments. This is where human ingenuity makes the fight against a disease possible. We discuss the different types of measures against infectious diseases next.
Essential for the eradication of diseases is the development of means to fight their spread – the second essential factor in the list above. Over the last centuries we have developed several means to fight the spread of infection and some of these have the potential to eradicate a specific disease entirely.
The appropriate measure to fight the disease depends on the type of pathogen and the way of transmission. For instance, antibiotics only work against bacteria, whilst anti-retrovirals only work against retroviruses. The way the disease is transmitted matters as well: if diseases spread via drinking water, filtering can stop the disease; if spread via mosquitoes, bed nets can stop the disease, etc.
The type of pathogen causing the disease is the first characteristic of a disease that informs the choice of appropriate measures against the disease.
These measures can be grouped into three categories, which are summarised in the figure below.
The appropriate measure depends on the specific disease. Both smallpox and rinderpest were eradicated using vaccines. However, vaccines have not been developed against every disease yet, so other methods like water decontamination, wound treatment and health education to block transmission can be used with great success as well, such as in the case of Guinea Worm (dracunculiasis).
Measures against the spread of disease4
Different transmission routes of the disease require different strategies
The list below – based on Checchi (2009)5 – describes the routes of transmission for all diseases discussed in this article (bold) and a number of other common infectious diseases.
Direct transmission routes (suitable for ring-vaccination)
- Air droplets: Inhalation or eye contact with infected droplets generated by breathing, sneezing, coughing. Less than 1 m distance required.
- smallpox, rinderpest, rubella, PPR (ovine rinderpest), whooping cough (pertussis), common cold, flu (influenza), meningitis
- Sexual: Transmission by unprotected sex.
- HIV, syphilis, chlamydia, gonorrhoea, hepatitis B
- Blood: Directly transferred from one person’s blood to another person’s blood, e.g. via unsafe injections or transfusions of unsafe blood.
- HIV, hepatitis B and C
- Mother to child (vertical): Transmission during pregnancy, childbirth or via breast milk.
- HIV, tetanus, syphilis, hepatitis B
- Any body fluid: Body fluids (blood, feces, vomit, breast milk, urine, semen) entering broken skin or mucosa (nose, mouth, vagina, anus)
- Skin-to-skin contact:
Non-direct transmission routes
- Airborn: Infection possible by entering a room an infected person was previously in or through air conditioning.
- tuberculosis, smallpox (rarely), measles, chickenpox
- Fecal-oral: Ingestion of faecal matter, facilitated by poor water, sanitation and hygiene conditions. Can be transmitted through objects.
- polio, most intestinal worms including pork tape worm, typhoid, hepatitis A and E, diarrhoeal diseases including cholera, shigella, salmonella, E. coli, rotavirus, amoebiasis, giardiasis
- Vector-borne: Pathogens undergo a life cycle inside humans as well as inside another “vector” species, typically insects.
- malaria (mosquito), Carrion’s disease (phlebotomine sand flies), lymphatic filariasis (mosquito), river blindness (Black fly), dengue fever (mosquito), Japanese encephalitis (mosquito), African sleeping sickness (Tsetse fly), leishmaniasis (Sand fly), schistosomiasis (fresh-water snail), typhus (lice, fleas, mites), relapsing fever (lice, ticks)
- Wounds: Deep cuts, infection of the umbilical cord after birth, animal bites
- rabies, tetanus
- Water, soil:
- Guinea worm (ingesting water), tetanus (infection through wounds, from soil), hookworm (walking barefoot on soil, or ingestion)
For those diseases that spread through direct contact between people – those listed under direct-transmission routes – the ring-vaccination principle can be applied. It allows to only vaccinate all people who came or will come in contact with an infected person, rather than vaccinating the whole population, to save money and time. This principle was successfully applied for the eradication of smallpox.6
Below we provide a summary of the eradication of Smallpox; you can find more detailed information in our much more detailed full Smallpox entry.
Smallpox was one of the most lethal and feared diseases in history7 caused by the variola major or minor viruses. It exclusively infected humans.
Symptoms included fever, vomiting, and the development of a rash covering the body that would turn into fluid-filled pustules. These pustules later formed scabs which eventually fell off to leave behind large and very visible scars.
Smallpox was a disease that spread via droplets when infected patients sneezed or coughed. The virus could more rarely spread via the clothes or bed sheets of infected people. Patients would be contagious for approximately 20 days, from the onset of the first rash until the last pustule falling off as a scab.8
While an infection of the variola minor virus would lead to death with a probability of less than 1%, the case fatality rate of the variola major virus has been estimated to be around 30%.9
Thanks to the early discovery of a vaccine and global efforts, smallpox was completely eradicated in 1977. It was the first and until today the only human disease to be eradicated.
Accounts of mankind's search for a means against smallpox date back as far as the accounts of the disease itself. A practice in China and India dating back to 1,000BC involved the nasal insufflation of dried and crushed scabs of smallpox patients' pustules.10
In England, a practice called 'inoculation' or 'variolation' became widespread in the 1720s which involved the injection of pus extracted from smallpox patients' pustules under an uninfected person's skin. The inoculated patient would usually suffer a minor smallpox infection and subsequently be immune against an infection.
Because both practices made use of the live variola virus the practices came with two serious disadvantages: First, for a certain time period inoculated patients were carriers of the disease themselves and could potentially further spread the virus. Second, the patients were at risk of actually contracting the disease and some died as a consequence of the procedure that was intended to protect them.
In 1796, the English doctor Edward Jenner discovered that inoculating his patients with the cowpox virus which was of the same family as the variola virus protected them against smallpox.11 Cowpox was a much milder and not lethal disease which made the procedure much safer for its recipients. It is the Latin word for cow, vacca, that gave rise to the name of this new practice: vaccination.
Smallpox was extremely deadly in the past. The data for London shown below suggests that 7.6 out of a 100 deaths in London prior to 1800 were caused by smallpox. In other words, around 1 in 13 deaths was due to smallpox.
As is typical for viral diseases against which no means existed, smallpox deaths fluctuated regularly over the course of a few years. This occurred because by the time the virus had infected a population, it either engendered immunity or death, so a renewed outbreak in a subsequent year would not infect as many people. The impact of Jenner’s invention of the cowpox vaccine in 1796 is clearly visible in the time-series as it had a remarkably fast and pronounced effect on the number of smallpox deaths in London from the beginning of the 19th century onwards.
However, it was not until 1980 that the World Health Organization would certify that the world had been freed of smallpox and ordered the destruction of variola virus samples in all but two high security labs. After its invention in 1796, vaccination still had to gain in prominence and improvements to the production and resilience of vaccine serum had to be made.
It was only with the establishment of the World Health Organization in the aftermath of World War II that international quality standards for the production of smallpox vaccines were introduced and the fight against smallpox moved from national to an international agenda. In 1966, the WHO launched the Intensified Smallpox Eradication Program. By then smallpox cases and deaths in Europe and North America had been driven down substantially but large parts of Asia and Africa still struggled under smallpox's disease burden.
The number of smallpox cases is shown by world region in the chart below. Shown here is the number of reported smallpox cases from 1920 until the last case in 1977. Even though smallpox had a high visibility and should therefore be relatively easy to document, the lack of an international organization dedicated to global health means the number of reported cases will possibly be substantially lower than the true total number of cases. Fenner et al. (1988) write "it is not unreasonable to regard the official figures reported to WHO as representing only 1-2% of the true incidence - probably nearer 1% for the years before the initiation of the global eradication programme."13 They go on by estimating that in the early 1950s - where the documented cases peaked with 546,000 cases in 1951 - there were actually 50 million smallpox infections every year and that by 1967 - a year in which 121,000 cases were reported - there were probably still 10-15 million infections around the world. We discuss the concerns with the data quality in our smallpox entry.
Initially, the WHO pursued a strategy of mass vaccination but soon realized that applying the ring vaccination principle (explained at the end of section II.2 above) by targeting the direct network of smallpox patients achieved a faster and more cost-effective reduction in smallpox cases. Finally, the last case of a variola major virus infection was reported in 1975 in Bangladesh, while the last infection of a variola minor virus occurred in 1977 in Somalia.14 In 1980, the governing body of the WHO, the World Health Assembly, officially certified smallpox as eradicated and recommended the end of all smallpox vaccination programs, 14 years after the start of the WHO Intensified Smallpox Eradication Program and 184 years after the invention of Jenner's vaccine.
The world map below illustrates from which year the variola virus was no longer endemic in a country.15 You can see that Somalia was the last country to eliminate smallpox in 1977.
Today, only two high security labs in the world (in Atlanta, USA and Moscow, Russia) still hold samples of the variola viruses for research purposes.
Rinderpest, also known as cattle plague, was a disease caused by the Rinderpest virus that infected primarily cattle and buffalo but was also found in zebus, water buffaloes, African buffaloes, eland, kudu, wildebeest, various antelopes, bushpigs, warthogs, giraffes, sheep, and goats.16
Symptoms suffered by infected animals included fever, wounds in the mouth, diarrhea, discharge from the nose and eyes, and eventually death. Death rates during rinderpest outbreaks were remarkably high, up to 100% in particularly susceptible herds.17
The virus spread via infected droplets, so by inhaling sick animals' breath, secretions or excretions. Rinderpest was a so-called dead-end disease for wild herds as their low population density inhibited the disease's spread. Together with the development of a potent vaccine in 1960, the dead-end in wild herds played an important role in achieving the disease's eradication in 2011. It is the first and until today only animal disease ever to be eradicated.18
While Rinderpest did not infect humans it severely affected them. Rinderpest outbreaks caused famines responsible for millions of deaths.19 The introduction of Rinderpest to Sub-Saharan Africa killed so much cattle that the landscape was permanently changed. For example, it allowed for the growth of vegetation that favoured the spread of the tse-tse fly. This fly transmits African sleeping sickness, a disease that still kills thousands every year and which occurred in major epidemics in the past.
The fight against rinderpest is an example for how case numbers could be driven down even before the invention of a potent vaccine, which was only developed in 1960. This illustrates that while having effective means against a disease was important for eradication, the proper implementation of second-best means significantly reduced the disease burden even before.
Before the development of a vaccine, quarantine, improved hygiene, slaughter and inoculation20 were common practices in containing rinderpest. The former two practices were effective thanks to rinderpest's transmission requiring close contact between infected and susceptible animals. Europe managed to achieve rinderpest elimination this way at the beginning of the 20th century, long before the introduction of the vaccine.21
Slaughter was another means to contain Rinderpest's spread. It was understandably less popular because all cattle had to be killed if one infected member was identified. Nevertheless, European Russia successfully eliminated rinderpest this way in 1908.
Inoculating cows with inactivated virus samples from infected animals was an idea inspired by the variolation practice against smallpox in humans. Thailand, the Philippines and Iran, for example, managed to eliminate rinderpest before the Second World War using inactivated virus samples from cows.22
In 1960, finally, English veterinary scientist Walter Plowright developed an inactivated vaccine – the tissue culture rinderpest vaccine, or TCRV – that induced lifelong immunity without major side effects or the risk of further transmission and could be produced at a low cost. His success was based on figuring out how to grow rinderpest virus in a laboratory outside of living organisms, using a method called tissue culture.23 In 1961, Albert Sabin used the same method to develop an oral polio vaccine.24 Plowright was awarded with the World Food Prize in 1999 for making rinderpest's "eradication, for the first time in human history, a practical objective".25
Rinderpest was only ever endemic in Europe, Asia and Africa (with two isolated outbreaks in Brazil in 1920 and Australia in 1923).26
In 1994, the FAO launched the Global Rinderpest Eradication Programme (GREP) with the goal of eradication by 2010.27 Thanks to the program's global surveillance and vaccination efforts (a ring vaccination strategy similar to that applied to smallpox was used)28, the last known rinderpest outbreak occurred in Kenya in 2001 with the last case being recorded in Mauritania in 2003. Over the next ten years, the GREP continued to search for rinderpest samples. Finding none, rinderpest was declared eradicated by the World Organization for Animal Health (OIE) on 25 May 2011.29
Unfortunately, no data on rinderpest cases and deaths seem to exist on a global level.30 The world map below illustrates that with the exception of the two isolated outbreaks in Brazil and Australia Rinderpest infections were limited to Europe, Africa and Asia. While Western Europe already eliminated Rinderpest successfully by the end of the 19th century, the last Asian case was recorded in Pakistan in 2000 and the last global case was documented in Kenya in 2003.
In 2014, 23 countries were reported to still hold samples of the rinderpest virus which is why the OIE and FAO aim to destroy most remaining rinderpest virus stocks and store a few remaining samples under international supervision in approved laboratories.31
The eradication of rinderpest from 1945 to 2011 is estimated to have cost the equivalent of 2017-USD 5.5 billion in 201732 but its economic benefits remain unknown. It is worth noting, though, that the 1982-1984 outbreak in most of Africa caused the loss of livestock of the equivalent value of at least 2017-USD 1.02 billion.33
Rinderpest and smallpox have demonstrated the feasibility of eradicating infectious diseases. The following table lists diseases for which there is hope that they could relatively soon be eradicated. They are listed in alphabetical order. If not otherwise referenced, the diseases are taken from Hopkins (2013).
In this entry we will discuss polio, Guinea worm and yaws as candidates for eradication. We will also discuss dog-mediated rabies as a disease for which campaigns are running to eliminate certain ways of transmission. Finally, we will discuss why the three most lethal infectious diseases – tuberculosis, HIV/AIDS, and malaria – are currently bad candidates for eradication.
Diseases suggested for eradication
|Disease||Neglected Tropical Disease (NTD)?||Elimination (but not eradication) target?||Infects only animals?||Discussed in our entry below?|
|Ovine rinderpest (peste des petits ruminants, PPR)||Yes||Yes|
|Pork tape worm (cysticercosis)||Yes|
|River blindness (onchocerciasis)||Yes|
Neglected Tropical Diseases (NTD) are a list of 20 diseases determined by the WHO “[…] that prevail in tropical and subtropical conditions in 149 countries – affect more than one billion people and cost developing economies billions of dollars every year. Populations living in poverty, without adequate sanitation and in close contact with infectious vectors and domestic animals and livestock are those worst affected.”34 Part of the WHO NTD roadmap is to eliminate as many of these as possible and to eradicate at least two of them by 2020, which should be Guinea worm and yaws according to their 2012 plan.35
Below we provide a summary of progress on Polio; you can find more detailed information in our full Polio entry.
Polio is short for poliomyelitis which is a disease that is caused by the poliovirus and infects exclusively humans.
Even though the disease is most known for its symptoms of permanently paralyzing certain body parts, 72% of infections actually lead to no visible symptoms. In fact, permanent paralysis only occurs in 0.5% of infections while in the remaining cases, symptoms are unspecific such as a fever or a cold or temporary paralysis. Polio can only lead to patients' death if their breathing muscles are paralyzed.36
Transmission of the virus occurs along the fecal-oral route, meaning infection occurs after ingestion of water contaminated with faeces from an infected person. The virus therefore spreads well in conditions of poor sanitation, for example when people defecate in the open or do not filter their water before drinking it.
Polio's relatively long incubation period of up to 10 days and almost three fourths of infections not showing any symptoms makes polio extremely difficult to monitor and the virus can spread for several months without being detected.
The poliovirus's transmission channel (in black) and means against it (in red)37
No cure exists that could reverse the permanent paralysis of a patient's body parts.
However, American scientists Jonas Salk and Albert Sabin discovered two effective polio vaccines in 1953 and 1961, respectively, which eliminated polio from the United States and Canada in 1979 and rapidly lead to a large reduction of the disease in Western Europe. While Salk's vaccine depended on injection with a needle, Sabin's vaccine could be swallowed. This made its distribution throughout the developing world much easier as no trained health workers were required for its administration.
Many developed countries' governments had already achieved great reductions in polio cases when the World Health Assembly founded the Global Polio Eradication Initiative (GPEI) in 1988. Inspired by the success of smallpox's eradication in 1977, the GPEI declared polio's eradication its mission. Since then, the organization has provided countries at risk of or still endemic with polio with support for national immunization campaigns and for monitoring and reporting systems.
Due to polio's spread via the fecal-oral route, it is unfortunately not suitable for a ring vaccination strategy like smallpox was. This is because the virus is able to survive outside the human body (in faeces, ground water, etc.) for several weeks and can therefore travel substantial distances before "needing" another human body for survival again. To break the chain of transmission of polio and to eventually eliminate it, a population needs to have a sufficiently high vaccination coverage which can only be achieved through mass immunization campaigns.
The chart below highlights the global decline of the estimated number of paralytic polio cases from 1980 onwards. In the peak year 1981, the number of paralytic polio cases are estimated to have exceeded 450,000 but were reduced to 43 cases in 2016 - a more than 100,000-fold reduction of paralytic polio cases. India has been the last country to eliminate polio in 2011, so in 2017 the polio virus was endemic in three countries only: Afghanistan, Nigeria, and Pakistan.
Guinea worm is a disease caused by the worm Dracunculus medinensis, therefore it is also called dracunculiasis. It is a vector-borne disease like malaria as both small water fleas as well as humans are part of the worm's life cycle. Only humans suffer from the infection, though. Guinea worms are found in South Asia and Northern Sub-Saharan Africa only.
Symptoms include painful blisters which are prone to bacterial infections that can cause permanent joint damage or ruptured tissue inside the body (which sometimes causes severe allergic reactions as well). However, it is rarely deadly.38
Humans become infected by drinking water contaminated with the worm’s larvae.39 Only when the worm surfaces at their human host's knee area, ten to fourteen months after ingesting the larvae, is a diagnosis possible.40
Thanks to improved water sanitation, Guinea worm cases were recorded in only two countries in 2017: Chad and Ethiopia.
The worm’s life cycle progresses from humans to water fleas (copepods) and back to humans and is visualized in the illustration below. In third stage larvae form, it enters the human host via contaminated drinking water. In the human gut, male and female worms mate. Subsequently, the female worms grow to a length of up to 120 cm and migrate to the lower part of the human leg to surface.
When the wound is washed, the worm releases numerous first stage larvae into the water. These find water fleas to infect from which they emerge as third stage larvae. Here, the life cycle begins anew with humans' drinking water contaminated with third stage larvae.
The transmission of Guinea worm disease via the Dracunculus medinensis worm (in black) and the available means against it (in red)41
The ways to prevent the spread of Guinea worm is included in the illustration above using a red font. As is the case with any vector-born disease, eradication can be achieved by interrupting the transmission from the vector to the human or the other way round ((2) Disease transmission in section II.2 above). In the case of Guinea worm, specifically, the worm depends on both water fleas and humans for survival so that there is more than one means against the disease. Sources of drinking water can be decontaminated and health education campaigns can inform about filtering drinking water and the appropriate disposal of removed worms.
Making water safe for drinking can be achieved by filtration; not washing wounds in public waters reduces larval spread. Public health education teaching people how to prevent the spread of Guinea worm has proven very successful.42
The WHO has been working to eliminate Guinea worm since 1981 and is hoping to eradicate it by 2020.43 Thanks to their effort, global data on the number of Guinea worm cases is available from 1986 onwards and is shown in the world map below. The limited geographic spread of the disease becomes especially clear when clicking on play at the bottom of the chart – it was only ever endemic in South Asia, Yemen and Northern Sub-Saharan Africa.
As it takes 10 to 14 months for the worm to emerge after infection, the last case will necessarily occur a year after the spread of the disease has been interrupted. To be certified Guinea worm free, a country needs to report zero cases for three consecutive years. After this period a country can apply for certification by the International Certification Team of the WHO to verify that transmission has indeed been interrupted. Currently, Kenya and Sudan are at their pre-certification stage, Chad, Ethiopia, Ghana and South Sudan are still endemic and Angola and the Democratic Republic of Congo are not known to have Guinea worm but are also still to be certified.44 However, only Chad and Ethiopia recorded a positive number of Guinea worm cases in 2017.
The line chart below illustrates the dramatic decline in the number of reported Guinea worm cases from two different data sources. From more than 892,000 reported cases in 1989, the number of reported Guinea worm infections dropped to only 25 infections worldwide in 2017 – 13 in Chad and 12 in Ethiopia.
For the early period especially the number of reported cases is much lower than the true number of cases. Greenaway (2004) reports an estimate of 3.5 million cases for the year 1986.45
Yaws, which is also called framboesia, is caused by the bacterium Treponema pallidum subsp. pertenue and classified as a Neglected Tropical Disease by the WHO. The bacterium infects humans and non-human primates. The name framboesia is derived from lesions that resemble raspberries (French: framboise).46
Within 90 days of being infected, patients develop symptoms in the form of lesions which completely disappear again within six months. A second round of lesions erupt months to years later, also healing off again within six months but likely leaving behind scars. In approximately 10% of untreated cases, a third round of lesions can occur and result in complications such as destruction of skin, bones and cartilage. The Oxford Textbook of Medicine describes yaws as "rarely fatal".47
Yaws is spread by skin-to-skin contact and rarely via objects, by bacteria entering through broken skin. An infection can spread from patients currently suffering from lesions, from people who carry the bacteria but at that moment do not have lesions, as well as from non-human primates.48
Yaws was almost eradicated in the early 1960s already but re-emerged due to discontinued support from the WHO and lack of attention by governments' healthcare systems. It is back on the WHO's list of diseases to be eradicated by 2020. This is believed feasible because effective antibiotics exist against yaws. The required mass treatments are relatively easy and cheap to administer. However, many countries do not monitor and report yaws cases to the WHO so that the global number of cases and thereby progress towards eradication remains largely unknown.49
The spread of the Treponema pallidum bacterium can be fought in two ways; improved hygiene and health education as well as antibiotic treatment. While improved hygiene and health education reduce the bacterium's transmission ((2) Disease transmission in section II.2 above), antibiotic treatment heals infected people and thereby reduces the number of carriers of the disease ((1) Infected Individuals in section II.2 above).50
Usually, yaws patients were treated with a single dose of penicillin but in 2012 it was discovered that a single, swallowed dose of another antibiotic called azithromycin could completely cure yaws.51
Yaws was usually diagnosed by the emergence of symptoms, i.e. the first round of lesions. Blood tests could confirm the presence of the Treponema pallidum bacterium but could not differentiate between active and past infections. Recently, new diagnostic tests have been developed that can diagnose a yaws infection faster and more accurately.52 This is important as the correct antibiotics could be immediately prescribed to confirmed yaws patients and avoid unnecessary oversubscription of antibiotics to patients suffering from similar symptoms caused by different pathogens.
The history of yaws eradication efforts illustrates the importance of political support, which we listed as the last of the five "features that make eradication easier" in section VIII. After all, yaws ticks all the other boxes of our five features: it is caused by only one subspecies of bacteria, only infects humans and non-human primates, good and cheap means against and diagnostics for yaws exist and both India and Ecuador have successfully interrupted transmission. The reason why yaws has not been eradicated yet is "simply" that for a long time, yaws has been neglected; no eradication campaigns were run and no records of case numbers were kept. Even today, only eight countries report yaws cases to the WHO.
The disease-causing pathogen of yaws was identified in 1905 but it was not until the establishment of the World Health Organisation in the aftermath of World War II that antibiotics were tested as a treatment option.53 Anecdotal evidence suggests that the disease burden prior to the WHO efforts was extremely high: In what today constitutes Ghana in 1936, 62.7% of all infectious diseases treated in government health facilities were yaws cases (for comparison, malaria only accounted for 20.3%).54 It has been estimated that in 1955, there were 50 million yaws cases worldwide.55
Therefore, after a few pilot projects that tested the efficacy of the antibiotic penicillin, the WHO together with UNICEF launched mass treatment campaigns in 46 countries in 1952. By 1964, after screening approximately 300 million people and administering approximately 50 million penicillin doses to patients and their close contacts, yaws cases were said to be reduced by 95% to just 2.5 million cases. In light of such a successful reduction in yaws's disease burden, internationally coordinated mass campaigns were discontinued and countries' primary health care systems were tasked with the elimination of the last 5% of cases. Global interest faded, developing countries' healthcare systems already had too much on their plate and since no records were kept anymore, nobody knew how yaws numbers were developing.
It was only with a WHO review of its Neglected Tropical Diseases in 2012 that yaws regained international attention. The WHO even declared it its goal of eradicating yaws by 2020.
These renewed eradication efforts make use of a different antibiotic called azithromycin (described in the previous section) that can be administered orally. A new strategy is to give azithromycin to everyone living in a yaws-prone community, regardless of whether they suffer from yaws or not.56
To achieve the eradication of yaws, better monitoring in endemic countries as well as determining the endemicity status of all remaining countries will be necessary. For instance, it is unclear whether the disease is still in circulation in the Americas today.57 The world map below visualised the only data available - a country's status of endemicity in 2016 and the number of cases for a few countries that reported yaws infections to the WHO. 14 countries are known to be endemic, even though for many more countries (all those shown in yellow) it is unknown whether yaws is still in circulation. Thanks to determined mass treatment and monitoring programs, India and Ecuador successfully eliminated yaws and are certified yaws-free by the WHO. They are coloured green in the map below. Endemic countries are depicted in red or orange in the map below, depending on whether the country reported the number of yaws cases to the WHO in 2016 or not, respectively. When hovering over the countries shown in red the number of cases will appear.
Dog-transmitted rabies is a disease for which the world currently aims for global elimination. This is because its eradication would require the immunization of all dogs and bats but because 99% of all human rabies cases are caused by dog bites, efforts currently focus on dog-transmitted rabies and are therefore categorized as elimination.
To start a global eradication campaign, it is helpful to have proof that a disease's elimination has proven possible – the fourth of our five features which suggest that eradication is possible listed in section VIII– and in the future even eradication might be possible.
Rabies is caused by seven different types of Lyssaviruses58 which infect all warm-blooded vertebrae including all mammals but is mostly found in dogs and bats. In humans and all prone animals, an infection attacks the nervous system, including the brain.
Dogs are the major reservoir for human infections and dog-transmitted rabies accounts for 99% of all rabies cases. All but one of the rabies viruses also infects bats. In countries where dog-transmitted rabies has been eliminated, like the Americas, the majority of the small number of remaining cases is now caused by bat bites.
Rabies in humans
Human symptoms include hallucinations, fear of water, spasms of the lung muscles and sometimes wild and aggressive behavior.
Rabies is transmitted via broken skin (typically wounds or bites) and mucosa59. Symptoms usually appear 20 to 90 days after the bite. Without treatment, rabies is 100% fatal caused by the malfunctioning of muscles important for survival, such as the lungs or heart.60
Rabies in animals
Symptoms in dogs are very similar, including pain at the site of infection, paralysis of the jaw, neck, and hind limbs, altered bark, snapping at imaginary objects, and producing a lot of saliva. Despite common belief of rabies causing dogs to become "mad" and aggressive, only a minority of dogs develops said furious rabies.61
Most wild animals lose their fear of humans when infected with rabies, some - especially cats - become aggressive.62 Infected bats experience disorientation, difficulty flying, behaviour changes including aggression, and their eyes take on a staring expression.63 With the exception of bats, untreated rabies is 100% fatal.
A first vaccine against rabies was prepared by Louis Pasteur and Émile Roux in 1885 from the brain of infected rabits.64 In 1974, said vaccine was replaced by a tissue culture derived vaccine, which is still used today and is more effective than the first vaccine.65
Typical treatment after exposure to a rabid animal involves cleaning the wounds and receiving a vaccine.66 The rabies vaccine has a very distinct feature that makes it unique: It is highly successful not only when applied before exposure to a Lyssavirus but also between the infectious bite and the onset of symptoms. This means that the vaccine acts as prevention ((3) Exposed individuals in section II.2 above) - which is what vaccines usually do - but also as treatment ((1) Infected individuals in section II.2 above). Once the symptoms have set on, however, death is basically guaranteed - only one person is known to have ever survived rabies without receiving the vaccine.67
While the treatment of infected humans with the vaccine is important for their survival, it does not constitute a means to fight the spread of the virus because humans do not usually transmit the disease. To eliminate dog-transmitted rabies (which makes up 99% of all rabies cases) one therefore has to vaccinate dogs as they are the carriers and transmitters of the disease. To reduce the number of infections, the United Against Rabies Collaboration recommends68
- to increase awareness to make quick treatment possible,
- to vaccinate dogs, (it is estimated that at least 70% of dogs need to be vaccinated to break the transmission cycle) and
- to treat humans after being bitten by a dog that could have rabies.
Wild animals such as foxes, coyotes and raccoons can be treated with oral vaccines in baits, distributed among wild animals by plane. No vaccine exists for bats. Current, but controversial, strategies to reduce the threat from bats is focused on vampire bats and includes destroying their settling places and poisoning the bats.69 The recent development of a vaccine against white nose syndrome in bats and of methods to deliver this vaccine to bats pave the way for other bat vaccines, such as against rabies.70
The United Against Rabies Collaboration was formed in 2012 with the goal to stop dog-transmitted rabies and consists of the WHO, the Food and Agriculture Organization of the United Nations (FAO), the Organization for Animal Health (OIE), and the Global Alliance for Rabies Control (GARC). Their campaign “Zero by 30” aims for having zero cases of dog-transmitted rabies by 2030.71
Despite the lack of a global elimination campaign by an international organization before 2012, rabies has declined by 73% since 1990. This is mostly due to reductions in South and East Asia, while Sub-Saharan Africa has only achieved a 26% reduction. The Americas and Europe had already been largely rabies-free at the start of monitoring in 1990.
The graph below shows the decline in deaths caused by rabies. Because rabies infections are 100% fatal, a graph of the number of rabies cases would be identical to the one depicted below. When you click on the tab "map" at the bottom you will see a world map where you can explore in which countries the deaths caused by rabies occurred. Most deaths occur in China and India which is in part due to their large population. In Africa most deaths occur in Nigeria and Ethiopia.
To achieve eradication, dog-transmitted rabies needs to be eliminated before all wild animals need to be cleared of rabies, too. Oral vaccines have eliminated rabies in red foxes in Europe and halted the spread of rabies in coyotes, foxes, and raccoons. The biggest challenge remaining after dogs, however, will be rabies in bats, as no vaccine exists for them yet.
“Today's categorization of a disease as not eradicable can change completely tomorrow, either because research efforts are successful in developing new and effective intervention tools or because those presumed obstructions to eradicability that seemed important in theory prove capable of being overcome in practice.”72
The three most lethal infectious diseases – tuberculosis causing 1.2 million deaths in 2016, HIV/AIDS causing 1 million deaths, and malaria causing 0.7 million deaths – are currently considered not eradicable because we do not have means against them which would be effective enough.73
The chart below shows the total number of deaths globally for these three diseases. Also shown is the death toll of rabies, the only other disease discussed in this entry that still causes patient's deaths.
A similar chart for the number of cases of infectious diseases can be found here. You can see that the number of annual malaria infections – 213 million in 2016 – makes all other diseases look small in comparison. Therefore, we have added a log/linear toggle at the top of the y-axis so that you can still reasonably compare the number of cases to each other.
Tuberculosis – often simply referred to as 'TB' – is caused by a bacterium from the Mycobacterium tuberculosis complex and can infect humans as well as many mammals and birds.74 It is the infectious disease that kills most people every year – 1.2 million in 2016.
An infection with TB bacteria only leads to the disease in 5-15% of cases, though the likelihood increases for smokers and HIV-positive patients. The WHO and the Institute for Health Metric and Evaluation (IHME) estimate that around one quarter of the world's population are infected with the bacterium without being ill.75 This is referred to as 'latent TB'.
Symptoms of active tuberculosis include a cough (often with blood in the sputum), fever, sweat, and weight loss. In the majority of cases tuberculosis infects the lungs but it can also affect other parts of the body.76 If patients with active tuberculosis are not treated, their chance of death is up to 70%.77 Patients' death is caused by the bacteria destroying patients' lungs which leads to suffocation.78
Tuberculosis is an airborne disease, which means that it is transmitted by exposure to the cough, sneeze or spit of an ill person. In other words, people with latent TB are not infectious. While both the number of people that become infected with the bacteria and the ones that develop active tuberculosis have been increasing, the number of deaths caused by the disease have declined since 1990 when 1.8 million people were killed by the disease. Whilst the currently available means against the disease (with sufficient investment) can feasibly reduce the number of TB cases and deaths to very low numbers (a 95% reduction from 2015 case numbers is targeted by 2035), the WHO and Stop TB Partnership note that means are not yet available to completely eradicate the disease. For complete eradication, research investment and development would be required for new tools to combat the disease.
Because an HIV infection weakens the immune system, patients are less able to fight off the spread of the Mycobacterium tuberculosis. Therefore, the chance of latent TB to transform into the active disease is 20 to 30 times higher for HIV/AIDS patients.79 Because of this people who are infected with HIV and die of tuberculosis are counted towards HIV deaths, not tuberculosis deaths in statistics.80 The WHO estimates that in 2016 approximately 40% of deaths among HIV-positive people were due to active tuberculosis.81 In other words, in people with HIV and tuberculosis the tuberculosis would most likely have stayed latent had it not been for the HIV infection. For more data, visit the section on Tuberculosis among People Living with HIV in our HIV/AIDS entry.
There are three existing ways to prevent the spread of tuberculosis which correspond to the three measures in our illustration in section II.2 above. Firstly, patients with active cases need to be isolated ((2) Disease Transmission) and treated ((1) Infected individuals). Soon after the start of treatment, patients are not infectious anymore which prevents the further spread of tuberculosis. The third method is preventative protection in the form of a vaccine ((3) Exposed individuals) called BCG (bacille Calmette-Guérin).
The first method, isolating tuberculosis patients, requires an accurate and speedy diagnosis, both of which is currently not available. The most common test is quick and cheap but misses roughly 50% of cases while the most accurate and currently available test requires 10 to 40 days for a diagnosis.82
The second method – curing patients with active tuberculosis – involves taking antibiotics, usually daily for 6 to 12 months. However, some strands of the TB bacterium have developed a resistance to many or all antibiotics currently available. This is known as multi-drug-resistant (MDR) and extensively drug-resistant (XDR) tuberculosis.
The third method, to prevent people from falling ill with TB in the first place, makes use of the only vaccine currently available, named BCG after its two French inventors Calmette and Guérin. Unfortunately, it only protects 50% of vaccinated people, a low effectiveness compared to other vaccines that typically protect 85-100% of vaccinated people.83 Research into a better vaccine is therefore ongoing.84 Additionally, people with latent TB can also be treated with antibiotics which kills the Mycobacterium tuberculosis they carry and prevents it from becoming active and contagious. The danger of treating people that are not sick with TB (yet) with antibiotics, however, is that it could make the problem of TB bacteria becoming resistant to antibiotics worse. Before prescribing antibiotics to latent TB cases, public health systems would therefore need to administer TB tests first to determine who is a carrier of the bacteria and who is not.
The WHO's End TB Strategy and Stop TB Partnership have set ambitious goals in addressing the global burden of TB: SDG Target 3.3 requires a 80 percent reduction in TB cases (and 90 percent reduction in deaths) from 2015 levels by 2030; by 2035, the goal is to achieve a 90 percent reduction in cases (95 percent reduction in deaths). To achieve this, progress rates will have to shift rapidly: current rates of a 1.5 percent decline in incidence per year would have to accelerate to 10 percent per year. The WHO estimates a US$2.3 billion per year funding gap to achieve this, but that the tools and treatments are available to achieve these targets with adequate funding.
Whilst its technically feasible to achieve the End TB targets of a 95 percent reduction in deaths and 90 percent in incidence (from 2015 levels) by 2035, new tools are needed if we are to completely eradicate this disease. The WHO estimates a US$1.2 billion per year funding gap in research for new TB-combating tools. Since the required tools for complete eradication are not available, we currently classify this as a non-eradicable disease.
As mentioned, bacterial resistance to antibiotics can prevent effective treatment. When a patient is infected with bacteria that do not respond to the two most powerful anti-tuberculosis drugs, rifampicin and isoniazid, their infection is classed as multidrug-resistant tuberculosis (MDR-TB). If on top of that, a patient is also resistant to a third type of common anti-tuberculosis drug, fluoroquinolones, and at least one alternative drug, a so-called second-line agent, the patient suffers from extensively drug-resistant (XDR) tuberculosis.85
In 2016, there were approximately 300,000 people worldwide that developed MDR-TB, which corresponds to 3.3% of all new active tuberculosis infections that year. While the number of new MDR-TB cases have more or less stagnated worldwide since 2000, the number of people with extensively drug-resistant (XDR) tuberculosis is on the rise with approximately 18,000 new cases worldwide in 2016 (0.2% of all new active tuberculosis infections that year).
The WHO's End TB Strategy is aiming to reduce the TB incidence rate by 90% and the number of TB deaths by 95% until 2035 relative to levels of 2015.86 To achieve these goals, they aim to develop new and better means against tuberculosis by 2025.
In 2015, the UN also included the fight against tuberculosis in Sustainable Development Goal 3. The goal is to "end the epidemic of tuberculosis (TB)" by 2030, but it is not clear what is meant with an 'end' of the epidemic. Neither the End TB nor the SDG 3 goal seems to aim for a complete eradication of the disease and instead the goal for the coming decades is a very significant reduction of the disease burden from TB.
The good news is that the world is moving in the right direction: The two largest global health statistics organizations, at the WHO and at the IHME, both agree that the number of deaths from TB declined by around one quarter since the year 2000 (see comparison here). In 2000, the WHO estimate was 40,000 deaths lower (1.69 million deaths estimated by the WHO vs. 1.73 million deaths estimated by IHME). This discrepancy has flipped for the latest numbers: in 2016, the WHO estimate was 90,000 deaths higher (1.3 million deaths estimated by the WHO vs. 1.21 million deaths estimated by IHME).
Below the IHME data is shown. The number of deaths per year has declined by 34% from 1990 to 2016. In absolute numbers that is a reduction from 1.8 million deaths per year to 1.2 million. Most of this reduction has been achieved in East Asia and the Pacific region.
Is the observed decline in the number of deaths due to fewer infections or due to higher survival rates among the infected? The graph below shows that both the number of people who are infected (prevalence of active and latent tuberculosis) and the number of new infections per year (incidence) have increased between 1990 and 2016. That means that the decrease in deaths from tuberculosis is not due to fewer infections but instead due to fewer infected patients dying of tuberculosis, as a result of antibiotic treatment.
For 2016 the IHME estimates that 1.92 billion people were carriers of the Mycobacterium tuberculosis, 25.7% of the world's population are affected by latent TB. 9.02 million of those 1.92 billion people became newly infected in that year (incidence) and 9.37 million people actually developed active tuberculosis (prevalence of active tuberculosis).
The very high prevalence means that the eradication of tuberculosis is currently a very difficult goal to achieve in the short- and medium-term. A more effective vaccine, better drugs to combat multi-drug resistant tuberculosis, and improved diagnostics would all be important steps towards controlling and eventually eradicating tuberculosis.
Below we provide a summary of progress on HIV/AIDS; you can find more detailed information in our full HIV/AIDS entry.
An infection with the human immunodeficiency virus (HIV) can lead to acquired immunodeficiency syndrome (AIDS). The HI-virus only infects and causes disease in humans.87
Symptoms differ by the disease's stage. A few weeks after being infected with the virus, flu-like symptoms combined with a rash on the upper body appear. Symptoms then disappear and for 8 to 10 years the number of immune cells constantly falls until it is eventually too low for the immune system to still function. The disease is then referred to as AIDS. Typically, it is not the HIV infection itself that proves deadly but secondary infections (e.g. the flu, tuberculosis) or cancer that the patient's weakened immune system can no longer fight off. In statistics, these deaths are nevertheless attributed towards HIV rather than the secondary infections.88
The virus is predominantly sexually-transmitted, but can also spread from mother to child during birth or breast feeding or through the sharing of injection equipment such as needles. The presence of the virus can be diagnosed cheaply and quickly with a blood test.
If detected early enough, HIV-positive patients can take drugs called antiretrovirals. Such an antiretroviral therapy (ART) suppresses the replication of the virus and thereby extends the lifetime of a patient's immune system, before the disease develops into AIDS. For ART to be effective, the drugs need to be taken every day. We showcase data on the availability of ART in our dedicated entry here.
Neither a cure nor a vaccine exist for HIV. Therefore, the only means to limit the spread of HIV that is currently available is to limit the (2) Disease transmission as explained in our illustration in section II.2 above, stopping the transmission from an infected to a healthy individual.
This includes the use of a condom for protected sex or sterile needles for safe injections. Health education or the provision of free condoms and needles can corroborate the effectiveness of this measure. HIV-positive expectant mothers can prevent transmission by adhering to antiretroviral therapy, delivering the baby by C-section and replacing breast milk with milk formulae.89 UNAIDS, the United Nations program on HIV and AIDS, collects data on condom use, mother-to-child transmission and education. These links take you to the respective sections in our HIV/AIDS entry where we have extensively visualized these factors.
HIV/AIDS currently cannot be eradicated because of the limited means to stop the spread of infections. For eradication to be achieved, the currently available means of health education and sterile equipment for wounds and injections would have to completely stop transmission before having to wait for all infected patients to die a natural death.
While the numbers of HIV/AIDS deaths and new infections increased during the 1990s, they have decreased since, thanks to increased public attention, funding and availability of antiretroviral therapy. While 3.13 million people became infected with the virus in 2000, it was only 1.87 million in 2016, a 46% reduction. HIV/AIDS deaths have declined from 1.91 million in 2005 to 1.03 million in 2016, a 40% reduction.
The chart below shows that the large majority of deaths from HIV/AIDS occur in Sub-Saharan Africa. To see the geographical breakdown by country, you can click on the "Map" tab at the bottom of the chart. The equivalent maps for the number of patients living with HIV/AIDS can be found in our dedicated entry here.
The WHO aims to reduce the number of new HIV infections to fewer than 500,000 per year by 2020.90 This is an ambitious goal as 1.87 million new infections were still recorded in 2016. Their strategy is to prevent infection at birth, decrease stigmatization and increase testing so that patients are aware of their HIV status to prevent the further spread of the virus. Another goal that does not contribute to containing the virus is to make antiretroviral therapy more widespread and cheaply available. Similarly to Tuberculosis, HIV/AIDS is also part of the UN's Sustainable Development Goal 3 which aims to "end the epidemic of HIV" by 2030.
Finding a cure or a preventative measure like a vaccine would be a big step towards eradication. Cured patients would no longer be carriers of the virus and a vaccine could protect the healthy from becoming infected. Simply making use of existing means such as protected sex or clean needle programs more effectively will not be sufficient to achieve eradication.
Below we provide a summary of progress on malaria; you can find more detailed information in our full Malaria entry.
Malaria is caused by single cellular parasites, from the genus of Plasmodium, most often Plasmodium falciparum. It depends on both mosquitoes and humans for various lifecycle stages, which makes malaria a vector-transmitted disease (discussed in section II.2 above).
Once infected a patient develops a fever, which sometimes occurs in waves, and flu-like symptoms. If untreated this can turn into severe malaria, which includes a long list of symptoms: lack of energy, abnormal bleeding, liver failure, collapse of blood vessels, breathing difficulties, seizures, unconsciousness.91 The exact mechanism of how patients die of malaria remains unknown.92
Malaria is transmitted to humans via the bite of a mosquito carrying the parasite. That means that a malaria-infected human typically does not infect other humans with the parasite.93 The varying life stages of a parasite is illustrated in the picture below and explained in the footnote.94 When an infected human gets once more bitten by a mosquito, the parasite travels back to the mosquito where it produces offspring and the lifecycle of the malaria parasite starts anew.
The parasite Plasmodium's transmission channel (in black) and means against it (in red)95
The majority of malaria cases occur in Sub-Saharan Africa and in children under 5 years old. Globally, every tenth child that died in 2016, died because of malaria. In Sub-Saharan Africa, this is true for almost every fifth child.96
Currently, malaria can be contained in two ways: (1) preventing people to be bitten by mosquitoes because they may carry malaria parasites ((2) Disease transmission in section II.2 above) and (2) curing infected patients so that even when they are bitten by mosquitoes the malaria parasite will be dead rather than re-enter the mosquito ((1) Infected individuals in section II.2 above).
The most common ways to prevent mosquito bites have been bed nets, larvicides and indoor residual spraying, where insecticides are sprayed onto walls where mosquitoes like to sit before biting humans.
Once infected with the malaria parasite, a patient can receive chemotherapy (this is different to the one against cancer) to be freed of malaria. This treatment is known as an antimalarial.
Despite decades of research, a vaccine against malaria does not exist. The main difficulty for vaccine development is that the parasite changes its appearance throughout its many life cycle stages.97
The World Health Organization launched the Global Malaria Eradication Programme in 1955. In 2000, malaria was included in the Millennium Development Goals (MDG): "Target 6.C: Have halted by 2015 and begun to reverse the incidence of malaria and other major diseases"98 which generated political awareness as well as increased funding. 795 million insecticide-treated bed nets were distributed from 2002 to 2016.99
The world map below illustrates how this increased effort translated into reductions in malaria prevalence: regions shaded in green are no longer malaria endemic today. You can find our own map of the country-by-country incidence of malaria from 1990 onwards here. The distribution of bed nets was responsible for 81% of the decrease in malaria cases.100
World map of past and current malaria prevalence101
Today, the majority of malaria cases and deaths occurs in Sub-Saharan Africa and children under 5 years old. Annual deaths have declined by 27% since 2005. With 720,000 deaths globally in 2016, malaria was the least fatal of the three currently non-eradicable diseases discussed in this entry. By clicking on "Map" tab at the bottom of the chart below, you can view an interactive world map of deaths by country.
The UN Millennium Development Goals (MDGs) featured a malaria reduction target; this was one of the few which were achieved by 2015. The UN Sustainable Development Goals also have a malaria reduction target. By 2030, it is their goal to "end the epidemic of malaria in all countries." The targeted level of reduction, however, is not clearly defined. Our SDG-Tracker contains further information and data on global progress towards the 2030 target.
Similarly, the WHO launched their Global Technical Strategy for Malaria 2016–2030 in 2015, setting ambitious and more specific goals. By 2030, among other goals, they hope to reduce the mortality rates and incidence of malaria by 90%.102
There is still a big funding gap in providing insecticide-treated bed nets in malaria-prone regions, estimated at $640 million for the period between 2018 and 2020.103 The total gap in funding that will be needed to reach the WHO malaria goals is estimated to be between $3.5 and $3.8 billion104 Additionally, the development of a malaria vaccine would make the global eradication of malaria more feasible as the parasite could be fought with several means (vaccine, bed nets, antimalarials) simultaneously.
The immediate benefit of eradicating a disease is obvious: preventing the suffering and saving the lives of people who would have been infected with this disease in the future.
But there are additional benefits from eradicating a disease:
- Removing a disease from the list of existing diseases makes doctor's lives easier and patient's lives better by reducing the chance of mis-diagnoses.
- No medication is needed to treat an eradicated disease. This slows the emergence of resistance. Take for example the (hypothetical) eradication of a bacterial disease. No more antibiotics will be used to treat cases (and suspected cases) of this disease. The reduced usage of antibiotics will slow the spread of resistance to antibiotics.105
- A disease that is eradicated can also no longer mutate or re-combine with other pathogens to form novel diseases.
- The final benefit is saving cost in the long term. In several ways eradicating a disease reduces expenditure in the future:106
- The direct effect is that the eradication reduces the costs of prevention and treatment.
- The indirect effect is that it decreases spending on diagnosing the disease and research into the disease.
- Lastly, there is also the effect of having a more healthy population that as a consequence is more productive.
Therefore eradication has economic benefits in the long term as more money is saved than the eradication campaign costs. This possibility is illustrated in the schematic comparison below.
Eradicating vs controlling a disease: schematic comparison of the costs
The eradication of a disease has several benefits as we have just seen. But whether it is worth spending money on a disease eradication program at a given time needs to be assessed for each disease separately.
There will always be other good causes we can spend money on. These include non-health causes, other health causes, the eradication of different diseases, and even research into more cost effective tools for eradication of a disease instead of eradicating it with existing means. The scenario or intervention which brings the highest benefit needs to be assessed for each disease separately. “Elimination and eradication are the ultimate goals of public health. The only question is whether these goals are to be achieved in the present or [by] some future generation.”107
There are five key characteristics which are not absolutely necessary but make eradication easier. Dowdle (1999) writes: “In theory if the right tools were available, all infectious diseases would be eradicable. In reality there are distinct biological features of the organisms and technical factors of dealing with them that make their potential eradicability more or less likely.”108
1. The disease is caused by only a small number of pathogens
Infectious diseases are caused by pathogens. These disease-causing microorganisms include bacteria, viruses, unicellular parasites, or larger parasites such as worms. We included them for all diseases discussed here in our summary table at the beginning of this entry. To eradicate a disease its pathogen needs to be eradicated. This means that diseases caused by one or a small number of pathogens are easier to eradicate than the ones caused by a larger number of pathogens. Examples make this clear:
- Smallpox, for instance, is caused by only two types of viruses that were eradicated in 1977 using a vaccine.
- Lung infections, on the other hand, are an example of a disease named after their shared symptom: infection of the lung. But these symptoms can be caused by a large number of viruses and bacteria. This makes the disease "lung infection" non-eradicable. Wound infections are an even more extreme example as they can be caused by many bacteria that would not be harmful to a healthy human with intact skin.
In other words, disease-causing organisms can be eradicated but not necessarily symptoms.
2. The disease has only one host
Diseases that only infect one species will be easier to eradicate than diseases that infect many species, i.e. that have "alternative hosts". Smallpox, for instance, was caused by a virus that only survived in and spread among humans.
Other diseases that have only humans as a host are:
- Guinea worm disease (dracunculiasis),
- pertussis (whooping cough), and
Many diseases unfortunately either rely on several hosts (vector-transmitted) or can even have several hosts as alternative reservoirs:
Malaria, lymphatic filariasis, and river blindness are examples of vector transmitted diseases. This means that the pathogen requires several hosts in its life cycle. The malaria parasite, for instance, needs both humans and certain types of mosquitos to survive. In a hypothetical scenario of all humans being cleared of malaria at the same time, mosquitoes could still carry the disease and re-infect humans. However, if all humans could somehow be protected from infections for a few months at the same time, the malaria parasite would eventually die out as all malaria-carrying mosquitos naturally have a short life span. If the malaria parasite was therefore not able to live on in humans before the malaria-carrying mosquitoes die a natural death, the parasite would be eradicated.
The influenza virus (which causes influenza or "the flu"), is an example of a pathogen with several alternative hosts. The influenza virus can infect humans, birds, pigs and other animals. It could survive in these animals even if all humans were somehow made immune. This means that even if a vaccine against all strains of the influenza virus existed, one would still have to immunize all humans, pigs and even wild birds to eradicate influenza.
Other examples of diseases with alternative hosts are
- rinderpest (infected cattle, buffalo, zebras, water buffaloes, African buffaloes, elephants, kudu, wildebeest, various antelopes, bushpigs, warthogs, giraffes, sheep, and goats);
- hepatitis A (which infects humans and other vertebrates);
- ebola (infects bats as well as some bigger mammals);
- rabies (infects all warm-blooded vertebrae including all mammals); and
- Japanese encephalitis (a so-called dead-end disease in humans, sheep, and cattle, which means that it is fatal and cannot spread, and is only transmitted among pigs).
While it makes eradication easier when there is only one host, we know it is not a requirement: Rinderpest was successfully eradicated in 2011 and it was a disease with alternative hosts that affected cattle, sheep and goats but also antelopes, buffaloes, deers, giraffs, wildebeests and warthogs. However, most cases occurred in domesticated animals so that eradication was achieved by vaccinating all domestic species in parallel.
Diseases that are vector transmitted will be easier to eradicate than those with alternative hosts, because one has to eradicate the pathogen from only one species for a sufficiently long time.
The fact that the pathogen is reliant on several hosts makes vector transmitted diseases easier to eradicate than diseases with several alternative hosts but harder to eradicate than diseases with just one host. In reality, more factors play a role in determining whether a disease is eradicable or not. Malaria, for instance, is a vector-transmitted disease but it is not currently eradicable because no good measures exist against it (the second point in the necessary factors for eradication in section II.1).
3. The disease is visible and good diagnostics exist
Good monitoring of outbreaks is essential for eradicating a disease as it allows us to contain an outbreak and prevent the spread of the disease.
If the disease has visible symptoms in the majority of infections or when a good diagnostic method exists, monitoring is easier. A smallpox infection, for instance, always led to visible pox and made monitoring simple. Pictures of infected people were shown and allowed health workers to find cases even in remote villages.109
Less visible diseases are harder to monitor. If symptoms only show in a minority of infections (e.g. polio) or if a diagnosis is difficult the disease can more easily spread unnoticed.
Another aspect that makes monitoring harder is if a disease is stigmatized. Sexually transmitted diseases for example might be hidden from the community intentionally.
4. Elimination has proven possible
It is promising if a disease has already been eliminated from certain geographical areas such as islands or even whole world regions.110 If, on the other hand, no country has ever eliminated a disease, it is likely that the current means against a disease are not effective enough yet to make eradication possible.111
Once a geographic region or indeed most countries worldwide have eliminated a disease, eradication becomes a feasible target. Currently there are three diseases that fall in this category: Polio, Guinea worm disease, and yaws. Because elimination of these diseases has been achieved these diseases are discussed in section IV: Eradication of other diseases above.
5. The perceived disease burden is high and financial, political and community support are available.
The perceived burden of a disease, the estimated cost of eradication, and the political stability of affected countries are further factors that determine the eradicability of diseases.
Polio is a good example here as it illustrates the powerful impact of both a unified international effort and local political support. In 1988, the Global Polio Eradication Initiative was set-up which has provided large-scale and, importantly, continued support for the eradication of polio. The number of paralytic polio cases has been greatly reduced such that in 2018 it is considered endemic in only three countries: Pakistan, Afghanistan, and Nigeria.
But polio also illustrates that positive developments might reverse. Nigeria’s case numbers, for instance, surged from 202 in 2002 to 1143 in 2006 because of suspicions that immunization campaigns were a cover for Muslim sterilization by the US government which lead to an 11-month boycott.112 The example of polio hence illustrates the importance of both international as well as local community support for eradication.
Eradication is the “permanent reduction to zero of the worldwide incidence of infection caused by a specific agent as a result of deliberate efforts”.113
Elimination refers to the “reduction to zero of the incidence of infection caused by a specific agent in a defined geographic area as a result of deliberate efforts.” A disease can be eliminated from a specific region without being eradicated.114
Infectious Diseases are “disease[s] caused by the entrance into the body of organisms (as bacteria, protozoans, fungi, or viruses) which grow and multiply there”.115
Communicable Disease are “infectious disease[s] transmissible (as from person to person) by direct contact with an affected individual or the individual's discharges or by indirect means (as by a vector)”.116
Vector-transmitted diseases are caused by pathogens that require several hosts in their life cycle. Examples include malaria (mosquito), lymphatic filariasis (mosquito), and river blindness (Black fly).
Ring-vaccination principle refers to a strategy of vaccination that targets merely the people who came or will come in contact with an infected person, rather than vaccinating the whole population ("mass vaccination treatment"). This principle was successfully applied for the eradication of smallpox.
Neglected tropical diseases are diseases determined by the WHO “[…] that prevail in tropical and subtropical conditions in 149 countries – affect more than one billion people and cost developing economies billions of dollars every year. Populations living in poverty, without adequate sanitation and in close contact with infectious vectors and domestic animals and livestock are those worst affected.”117
- Description: A good summary of the methods available to eradicate Ginea worm disease, polio, malaria, lymphatic filariasis and river blindness. Figure 1 of the paper is especially helpful as an illustrated summary.
- Date of publication: 3 January 2013
- Available at: http://www.nejm.org/doi/full/10.1056/NEJMra1200391
Online publication The History of Vaccines
- Description:This web publication is brought out by The College of Physicians of Philadelphia and describes disease eradication in general (see link below) but also on specific vaccine-preventable diseases (found in the table of content on the left) in an accessible and interactive way.
- Date of publication: Last updated 25 January 2018
- Available at: https://www.historyofvaccines.org/content/articles/disease-eradication
- Description:This article discusses how vector-borne diseases, specifically with insects like malaria, can be controlled, eliminated or even eradicated.
- Date of publication: 24 November 2017
- Available at: http://science.sciencemag.org/content/358/6366/998
Vaccinating wild animals
- Description:This article describes efforts to vaccinate wild animals which can be used for the eradication of diseases with alternative hosts like rabies.
- Date of publication: 22 March 2018
- Available at: https://www.sciencenews.org/article/oral-vaccines-could-save-ethiopian-wolves-extinction
We provide all data sources for this entry on a separate page.