Japanese journal of infectious disease

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The introduction of vaccines has had a major impact on preventing deaths caused by infectious diseases and improving the life-quality of millions of people. Some diseases that can be effectively controlled with vaccinations cannot be eradicated. The terms eradication and control do not have generally accepted definitions (Miller, 2006). For the purposes of this essay I will define eradication as an attempt to achieve zero cases of infectious disease worldwide and to destroy the causative agent in the nature (Miller, 2006). Disease control is a local ongoing process to stop the pathogen transmission, with the ultimate goal to eradicate the infectious disease (Barrett, 2004). Infectious diseases that can potentially be eradicated must meet certain criteria, such as human to human transmission, accurate diagnosing of the condition and, demonstration that transmission can be stopped in different geographic areas (Orenstein, 2007). However, there are several factors that hinder both the eradication and control of infectious disease, such as biological-technological feasibility, economic considerations and political-societal support (Aylward et al, 2000). The aim of this project is to discuss if effective disease control is currently a better method to keep infectious diseases in check as opposed to eradication by looking at the successful eradication of smallpox and the ongoing effort to eradicate poliomyelitis.

Criteria for eradication

Firstly, it is important to understand the criteria any disease needs to meet to be considered for eradication. Miller (2006) has proposed two categories of scientific and economic factors relevant for permanently interrupting transmission of infectious disease. The scientific factors include: the chain of transmission must only include humans and pathogens causing the disease must not have a natural reservoir, vaccinations for the disease must confer long-term protection, the ecology of pathogen must be very well known and effective laboratory confinement must be possible. Additionally, from economic aspect investments into eradication programme must pay off to attract investors and funding (Miller, 2006). However, as Miller points out, any eradication campaign is risky because at the start it is impossible to estimate if there is going to be enough funding and political support. Furthermore, the feasibility of eradication differs in developed and developing countries due to overall health level, political unrest, infrastructure and communication services, and financial situation (Miller, 2006). Thus, even if a disease meets the criteria and can be a candidate for eradication it will not be guaranteed that eradication efforts lead to success.


In the last 100 years there have been several attempts to eradicate different diseases. However, the only successful eradication campaign so far has been that of smallpox. Prior to eradication the outbreaks of smallpox were frequent in many parts of the world, however, by 1950 it was eradicated from most developed countries (Fenner, 1982). The eradication campaign of smallpox was announced by the World Health Organization (WHO) in 1967 (Henderson, 1977) and it is one of the greatest success stories of 20th century medicine.

Origins and characteristics

Smallpox is a severe infectious disease, caused by variola virus, which is a genetically stable DNA virus (Nomoto et al, 2002) that spread from Asia to Europe and Africa at the beginning of Christian era (Henderson, 1977). There are two different types of smallpox, variola minor and major, the latter being more dangerous with 30% of death rates (WHO). The airborne variola virus (Henderson, 1977) spread among humans via inhaling and had an incubation period of 7-17 days, followed by severe pains, fever and other flu-like symptoms that made the victims stay at their house, confining the cases of smallpox into limited areas. Smallpox is contagious only during the time when victims have rash, which develops on face and extremities. Rash is characteristic to smallpox and it is easy to diagnose quite accurately. People who have recovered from smallpox have a lifelong immunity against the disease (Henderson, 1977).

Vaccines and efficiency

Smallpox does not have any known treatment besides vaccination (WHO). Before Collier developed heat-stable smallpox vaccine in 1950s only liquid vaccines were available, which were restricted in their use due to heat-sensitivity (Henderson, 1977). The new freeze-dried vaccine proved to be very efficient since only one dose had immediate effects and conferred long-lasting immunity (Arita et al, 2004). In addition (Henderson, 1977), the vaccination process itself was easy and the re-use of needles reduced the cost of vaccination. Most developed countries were already vaccinating people before the start of WHO campaign and had stopped the transmission in their country (Henderson, 1977). The first initiative to eradicate smallpox came from Soviet Russia and the USA; their proposal was approved by World Health Assembly (WHA) in 1966 and the WHO campaign officially began in 1968 (Foege, 1998).

Reasons for successful eradication

Firstly, smallpox met the criteria of eradicable diseases and had several biological features that facilitated its eradication. Fenner (1982) suggested that the severity of the condition, easy identification by people without medical qualifications or training, human to human transmission, limited spread, the absence of recurrent infectivity and the lack of animal reservoir had a major impact on the eradication effort. Furthermore, the availability of effective heat-stable vaccine that was either made by local governments or donated made the vaccine cheap and sustainable in different socio-environmental conditions (Henderson, 1982). Secondly, there was a strong political support for the campaign: all developed countries that had already eradicated the disease had to make huge investments to maintain the quarantine programmes and nation-wide immunizations because air-travelling could import smallpox from an endemic country and release the pathogen. Thus, they had economic interest and motivation to finance the eradication elsewhere because if eradication was certified in all countries then the investments and vaccinations could stop and so could all future expenses related to smallpox (Fenner, 1982). Thirdly, although the epidemiology of smallpox was important to the eradication effort, the campaign might have failed without the constantly improving reporting-surveillance methods, engaging of local health workers and community members, and, close link between field workers and research labs to improve the vaccine (Henderson, 1977). Also, the programme had strong central leadership yet it was very flexible and suited to the needs of each country, which ultimately led to success (Henderson, 1977). Finally, the eradication campaign lasted only 10 years, which is not lengthy enough to discourage people in the feasibility of the endeavour. The last endemic case of smallpox occurred in 1977 in Somalia, and the last case of smallpox transmission took place in England in 1978 when variola virus was accidentally released from a medical school lab (Hull et al, 2001). This incident points out potential problems of containing viruses and preventing them from causing new outbreaks. The certification of smallpox eradication was given in 1980, 2 years after the last case of smallpox (Arita et al, 2004).

Setbacks and last efforts

The success of smallpox eradication does not mean that it was easy to achieve. The program struggled with logistic issues, lack of money and inconsistent political and societal support (Arita et al, 2006). India and Ethiopia (Fenner, 1982) were the last countries where smallpox was endemic, however, the methods applied in these countries proved to be very effective. In 1973 in India the aims of the program changed from mass vaccination to surveillance and containment; also, weeklong searches were carried out to detect new cases. Reward for reporting a case together with easy detection helped to stop the transmission by 1975, while at the start no one believed it was possible. Intensive campaign in Ethiopia started in 1971, where mountains, climate and dispersed population had hindered previous vaccination efforts. Increasing the number of health workers, WHO investments into improving transportation and house to house searches helped to reach more people living in remote areas and led to the last reported case in 1976 (Fenner, 1982). Thus, the intensified efforts to eradicate smallpox from its last strongholds were fruitful and led to the campaign to a successful end. The WHA declared smallpox eradication in 1980 and vaccination activity had stopped in all countries by 1984 (Jezek et al, 1987).


After the great success of smallpox eradication, the WHO declared a global campaign against poliomyelitis (polio) in 1988 (Hull et al, 2001). Polio is a good candidate for eradication due to its crippling effects, especially in children. It also fits the criteria of eradicable diseases suggested by Aylward and collaborators (2000) and Miller (2006): transmission occurs only between humans and it lacks an animal reservoir, there is an effective orally administered polio vaccine, the severity of the disease can be prevented by immunization and reducing or stopping immunization will save millions of dollars every year; additionally, the strong collaboration and support from the Rotary International, UNICEF, WHO, Centres for Disease Control and Prevention (CDC) and governments has provided wider societal and political support. There has been significant success after 20 years of efforts trying to eradicate polio: 350000 polio cases and 125 endemic countries in 1988 have been reduced to 1997 cases and 4 endemic countries by 2006 (Dutta, 2008). However, polio eradication has proven to be more difficult due to unforeseen factors concerning the vaccine and the last strongholds of polio that seem impossible to dismantle; as a result many people have lost faith in the campaign and think that effective control may be a better alternative (Roberts, 2006).

Origin and Characteristics

The antiquity of polio is widely acknowledged and Egyptian stone carvings dated to ca 1500 B.C. depicting a man with limb deformities is believed to represent an early case of polio (Sass, 1996). Polio (Nomoto et al, 2002) is a paralytic disease caused by poliovirus, which is a genetically unstable RNA virus and belongs to the family of enteroviruses. It has three wild serotypes that are causative of polio. In developing countries it affects 1 in 200 people, whereas only in 1% of cases the virus invades the nervous system and causes paralysis (Nomoto et al, 2002). The victims shed virus with their faeces for weeks or more after vaccination, thus, the virus can sneak back and circulate in a community without being detected (WHO). Poliovirus spreads mainly via oral-faecal transmission and replicates in the gut and throat (Roberts, 2004). The disease has the most devastating effect in children under 5 (WHO). Furthermore, most cases of polio have atypical symptoms, which hinder the diagnosis and containment of the disease (Arita et al, 2006). Due to the low specificity of polio symptoms the surveillance mechanisms focus on acute flaccid paralysis (AFP) in under 15-year-olds and to confirm the condition stool samples need to be collected and analysed in the lab (Hull et al, 2001). Due to lengthy and complicated identification process gives the pathogen an opportunity to re-enter the environment and cause new outbreaks of polio.

Vaccines and efficiency

Similarly to smallpox, there is no cure for polio and vaccination is the only way to prevent the disease (WHO). There are two types of vaccines available: inactivated polio vaccine (IPV) developed by Salk in the 1950s and oral polio vaccine (OPV) that contains live virus advanced by Sabin in the 1960s; both have advantages and disadvantages (Hull et al, 2001). OPV is widely used because it provides intestinal immunity, is cheap to produce, easy to administer, and spreads from person to person conferring immunity to unvaccinated individuals in proximity (Miller, 2006). At least 3 doses of OPV in infants are required for the vaccine to work; however, studies have shown that 3 rounds of OPV provide only 70-80% immunity in some tropical settings but often the vaccine coverage is low, leaving many children unprotected and triggering outbreaks due to uneven population immunity (Hull et al, 2001). In addition, the efficiency of vaccine is dependent on the general health situation of the population, which in many developing countries is low; for example 10 doses of OPV are insufficient against polio in some regions of India, whereas only 2 doses were effective in Japan (Dutta, 2008). Children who live in poor sanitation conditions often have chronic diarrhoea that undermines vaccination since the medication leaves the body before it could have any effects (Roberts, 2004). A major problem with OPV is its heat sensitivity: polio serum needs to be kept below 8 degrees Celsius, which can be difficult in regions with hot tropical climate and civil disturbance (Westhead, 2009). This problem is being addressed and currently all OPV acquired through UNICEF have thermo-sensors that monitor heat exposure of the vaccine and allow limited use in the heat without compromising the vaccine (Hull et al, 2001). Furthermore, virus strains in OPV can mutate and regain the ability to circulate and may cause vaccine-associated paralytic polio (VAPP), which occurs rarely and has low transmission rates (Minor, 2009). Some people with compromised immune-system may shed the vaccine derived poliovirus (VDPV) for years, acting as a reservoir for the disease and current medicine has no cure to stop it (Miller, 2006). Thus, despite the cost-efficiency ratio of OPV it has some major faults that may prevent the achievement of eradication. On the other hand, IPV is administered via injection, making it expensive to use, especially in developing countries (Miller, 2006). IPV does not cause VAPP or VDPV, does not spread from person to person, does not provide intestinal immunity and does not prevent oral-faecal transmission of the disease; thus, IPV requires higher vaccination coverage (Nomoto et al, 2002). To address the problem of VAPP and VDPV it has been suggested to switch from OPV to IPV , however, the high cost of IPV makes its use in developing countries unlikely but as long as OPV is administered the eradication of polio cannot be conclusively confirmed (Minor, 2009).

Efforts and methods to defeat polio

Firstly, by 1990s 80% of children were vaccinated against 6 diseases providing high overall immunity across populations and the feasibility of eradicating polio was demonstrated in different geographic areas across countries with different economic status (Hull et al, 2001). The methods of eradication were inspired by the success of Pan-American Health Organization programme that eradicated polio from the Western hemisphere by 1990 (Hull et al, 2001). By today US$ 4 billion has been invested in the campaign and the global cases have been reduced by 99% since 1988 (Roberts, 2006). An estimated US$ 1.5 billion will be saved every year if polio eradication is finished and vaccinations ceased (Aylward et al, 2000). Hull and collaborators (2001) have summarised the strategies used to eradicate polio: Firstly, routine immunizations that aspire to deliver 3 OPV doses to infants during their first year of life; secondly, the introduction of national immunization days (NIDs) rapidly boosts the immunity of under 5-year-olds and must be continued for at least few years after the last reported case; thirdly, the surveillance targets cases of AFP and sends tool samples for analysis; and finally, mopping-up immunizations are intensive house to house searches that reach the majority of people at the end phases of polio campaign (Hull et al, 2001). Recent discoveries allow OPV to target type-specific poliovirus, either type 1, or 3; the use of monovalent OPV (mOPV) may increase the performance of vaccine in fighting polio in countries where the disease has been difficult to tackle so far (Aylward et al, 2006). Poliovirus type 2 had already been wiped out by 1999, convincing governments and financiers that achieving eradication of polio is possible (Roberts, 2006). mOPV targeting type 1 poliovirus has already been effectively used in several regions in Egypt and India (Aylward et al, 2006). Additionally, leaders of polio campaign have negotiated truces for vaccinating children in war zones (Tangermann et al, 2000). The initial goal of polio campaign was to stop vaccinations after the eradication has been certified, for this to happen, additional efforts are required: safe containment of poliovirus stocks with intensive surveillance until 3 years has passed from the last polio case to qualify for certification; and, the final step is the stop of vaccination (Hull et al, 2001). Post-eradication vaccination should be carried out with IPV to avoid shedding of virus and has to reach high coverage rates (Dutta, 2008). The last steps of polio eradication are problematic and may be impossible to accomplish in current political situation.

Difficulties and setbacks

In addition to the previously mentioned problems, polio eradication campaign has faced serious setbacks that have caused doubt in the likelihood of polio eradication. One of the first defeats was the polio outbreak in Hispaniola in 2000, caused by VDPV while the island had been known to be polio-free for years and vaccination rates had dropped below 30% (Roberts, 2004). In 2003 polio vaccinations were ceased in Nigeria because the Muslim leaders feared that the vaccine was contaminated and would make the population infertile; as a result polio spread to more than 18 previously polio-free countries (Kluger, 2005). Vaccinations were restarted in 2004 after all the tests for contamination were negative but by then polio cases had skyrocketed and 20% of children in Nigeria were left unprotected (Kluger, 2005).The fact that the virus can be hard to detect was acknowledged in 2005 when poliovirus had been silently circulating in Sudan for years (Roberts, 2006). Moreover, wars challenge polio eradication because entire countries are inaccessible, dangerous for health workers and vaccinations cannot be regularly carried out. After the declaration of the World Summit for Children in 1990 days of tranquillity were introduced in order to to vaccinate children in conflict areas and provide them with basic health care (Tangermann et al, 2000). Also, since the countries affected by conflict usually lack the resources to cover the costs of vaccination, money has to come from external sources (Tangermann et al, 2000). War activity often forces thousands of people leave their home and move to refugee camps where conditions for the spread of infectious diseases including polio are favourable. The mobility of refugees can undermine the vaccination campaigns because unvaccinated people may reintroduce the virus into a pathogen free area. Moreover, the dangers concerning poliovirus containment must be addressed: the stocks of poliovirus are distributed among numerous labs and there is currently no central high-security storage facility; also, the small size of poliovirus genome with known sequence can be synthesized in lab and used against susceptible populations by terrorists (Dove et al, 1997). Even without human interference the unstable poliovirus could mutate and combine with other enteroviruses in nature (Arita et al, 2004). Finally, the 4 endemic countries India, Pakistan, Afghanistan and Nigeria are populous, densely habituated, have poor sanitation and many regions that are difficult to access, and thus, have rendered all efforts to tackle polio useless (Aylward et al, 2006). Even if polio eradication was to be achieved it is questionable if vaccination will ever stop in the western world, especially after the shock of 9/11 and in the growing threat of bioterrorism (Roberts, 2004).

Criticism of polio eradication campaign

The critique of the campaign is related to the issues concerning vaccine related polio cases, difficulties of surveillance due to atypical symptoms, disturbance of routine immunizations caused by conflicts and increasing indifference of people due to delays; many of these aspects have been discussed in previous sections. I will now look at the more general statements against polio eradication programme that argue why all future eradication efforts need reconsidering. One of the main arguments against the campaign is that eradication of a single disease can only be done at the expense of other health resources; even with international help developing countries need to divert significant amount of their own resources that could be used for providing other health care services (Arita et al, 2004). Henderson points out that in developing world polio only affects 1 in every 200 and kills 1 in 2000 people, thus, polio is not as big threat in comparison to other infectious diseases and would otherwise not be considered a priority (Roberts, 2004). Others have suggested that money and attention should be focused on major killer diseases, such as AIDS and malaria that kill, respectively, 3 and 1 million people every year (Kluger, 2005). The initial polio eradication deadline of 2000 was not kept and new deadlines are constantly delayed, the most recent one in 2015 (Arita et al, 2006). 20-year-old ongoing polio campaign causes frustration and lack of interest, using up millions that could be used for increasing the overall health status in people living in developing countries rather than trying to eliminate one of many diseases. The annual costs to maintain polio campaign have increased from circa US$ 350 million to US$ 700 million in 2005 (Roberts, 2006) yet there is a significant chance that vaccinations will not stop after eradication has been certified (Arita et al, 2004). Finally, the conditions for polio eradication have changed after the eradication of smallpox: world population has increased from 4 billion in 1977 to 6.2 billion today; and political situation has become more complicated after the end of Cold War when two superpowers could dictate their will (Arita et al, 2006). Reconsidering the future of eradication does not mean that infectious diseases are not worth the money and effort. As Miller phrases it: "choice is not between doing nothing and eradication but between optimal level of control and eradication" (Miller, 2006).


The aim of this paragraph is to discuss if effective disease control could replace any future eradication efforts, including the ongoing polio campaign. The current idea of polio eradication is based on the success of smallpox campaign, however, Arita and collaborators (2004) argue that eradicating poliovirus in humans and in the environment is impossible today and propose redefining the term 'eradication' so that it would only encompass the determination of pathogens in humans. Furthermore, Caplan (2009) argues that eradication is risky because if eradication is achieved and all immunization activity stops then humans will be left vulnerable to the possible re-emergence of the pathogen. As the Hispaniola incident proves we can never be certain if a virus has gone forever or not. In 2005 the WHO approved the Global Immunization, Vision, and Strategy (GIVIS) programme that puts more emphasis on providing health care to more people and tries to integrate immunization with other forms of medical service (Arita et al, 2006). Sceptics argue that the WHO new policy prioritizing the development of basic health care facilities network may be lethal to polio campaign due to decreased political interest and support (Roberts, 2006). In order to eradicate an infectious disease the vaccination coverage has to be high, which leads to another significant problem. The patients can choose not to be vaccinated but by doing so they pose a risk for others and stand in the way of eradication (Caplan, 2009). It will be impossible to deny patients their right to refuse from treatment especially in times when people are more aware of the possible effects of vaccines and their rights to control what will be put in their body; also, many will refuse from vaccination due to religious reasons. Thus, considering all of the above, it has been suggested that eradication should be replaced by effective disease control (Arita et al, 2006).

Arita and Henderson are the two major opponents of polio eradication and they believe that the campaign in its current form cannot succeed; they suggest the incorporation of polio vaccine into routine immunization to maintain the accomplishments made so far (Roberts, 2006). In case of effective disease control all measures used to fight polio will be maintained until worldwide cases drop below 500 in less than 10 countries, after which the emphasis will be on surveillance and vaccinations will be carried out indefinitely (Arita et al, 2006). Another important concern is money: polio eradication campaign needs at least a few more billions to complete the task; however, the feasibility of eradication is uncertain (Arita et al, 2006). The opponents for polio campaign argue that increasing investments to achieve the goal is worth it and will save money in long term (Chan, 2007). Polio eradication from the last pockets is expensive if not impossible. Nevertheless, the improvements made with mOPV vaccines and concentrated efforts to tackle the last regions where polio is endemic cannot be ignored. If polio eradication programme stopped now then money and effort invested in the campaign would be lost, also, exposure to poliovirus at an older age is more dangerous (Miller, 2006). However, after the polio eradication programme reaches closure any further plans to eradicate a next disease should be postponed. Smallpox and polio campaigns started off with little knowledge about the epidemiology of the disease and incomplete strategy. No other disease should ever be considered for eradication without extensive research and thorough analysis. Furthermore, chasing down one disease does not relive the disease burden that hinders people in developing countries. Investing in improved broad base health care that is accessible to everyone will help to increase the overall healthiness in developing countries and in the long run will improve the life quality of millions of people.


To sum up, it can be said that all future eradication campaigns have to be carefully assessed before the start of execution. The epidemiology of a disease that is considered for eradication plays a key role in the success of any campaign, together with the availability of effective stable vaccines. The criteria of eradicable disease were fulfilled by both smallpox and polio, however, only smallpox campaign managed to achieve the goal of zero cases in humans and destruction of pathogen in the environment. In addition, in current political climate bioterrorism is regarded as a considerable threat to international security, therefore, stopping vaccination even after eradication may not be possible since it would leave humankind unprotected against the re-introduction of pathogens. Also, reaching an international agreement to start a project with high commitment may be currently impossible due to the clashing interests of developing and developed countries, yet, political support and funding are crucial for any global undertaking. Thus, at this point effective disease control instead of eradication may be more feasible and cost efficient.


  • Arita, I., Nakane, M., Wickett, J., 2004, Eradication of infectious diseases: its concept, then and now; Japanese Journal of Infectious Disease, Feb;57(1):1-6.
  • Arita, I., Nakane, M., Fenner, F., 2006 , Public health. Is polio eradication realistic?; Science, May 12;312(5775):852-4.
  • Aylward,B., Hennessey,K.A., Zagaria,N., Olive,J.M., Cochi,S., 2000, When is a disease eradicable? 100 years of lessons learned; American Journal of public Health, Vol.90, No.10.
  • Aylward ,R.B., Maher, C., 2006, Interrupting poliovirus transmission-new solutions to an old problem; Biologicals: Journal of the International Association of Biological Standardization, 2006;34(2):133-9.
  • Caplan, A.L., 2009, The art of medicine: Is disease eradication ethical?; The Lancet, October 3, Vol.374, No.9696.
  • Dove, A.W., and Racaniello, V.R., 1997, The polio eradication effort: should vaccine eradication be next?; Science 277:779-780.
  • Dutta, A., 2008, Epidemiology of poliomyelitis-options and update; Vaccine, 26(45):5767-5773.
  • Fenner, F. 1982, A successful eradication campaign. Global eradication of smallpox; Review of Infectious Diseases 4916-930.
  • Foege, W.H., 1998, Confronting emerging infections: lessons from the smallpox eradication campaign; Emerging Infectious Diseases, 4:412-413.
  • Henderson, D. A., 1977, Smallpox Eradication; Proceedings of Royal Society of London, October 19, 199:83-97.
  • Hull, H.F, Aylward, R.B., 2001, Progress towards global polio eradication; Vaccine, 19(31):4378-84.
  • Jezek, Z., Khodakevich, L.N., Wickett, J.F.,1987, Smallpox and its post-eradication surveillance; Bulletin of World Health Organization, 65(4):425-34.
  • Kluger, J., 2005, Polio's back. Why now? The fear of vaccines helped the virus spread from Africa to Indonesia. Could it reach the U.S.?; The Time Magazine, May 16;165(20):46-7.
  • Miller, M., Barrett, S. and Henderson, D. A., 2006, Chapter 62: "Control and Eradication." in Disease Control Priorities in Developing Countries (2nd Edition),ed., 1 163-1 176. New York: Oxford University Press.
  • Minor, P., 2009, Vaccine-derived poliovirus (VDPV): Impact on poliomyelitis eradication; Vaccine, May 5;27(20):2649-52.
  • Nomoto, A.; Arita, I., 2002, Eradication of poliomyelitis; Nature Immunology, 2002;3(3):205-8.
  • Roberts, L., 2004, Polio: the final assault?; Science, 2004;303:1960-8.
  • Roberts, L., 2006, Global health. Polio eradication: is it time to give up?; Science, May 12;312(5775):832-5.
  • Sass, E. J. with Gottfried G. and Sorem, A., 1996, Chapter 1: Poliomyelitis: A Brief History in Polio's Legacy: An Oral History, Lanham, Md.: University Press of America
  • Tangermann R H; Hull H F; Jafari H; Nkowane B; Everts H; Aylward R B, 2000, Eradication of poliomyelitis in countries affected by conflict; Bulletin of the World Health Organization, 78(3):330-8
  • Westhead, R., 2009, India 'racing' to wipe out polio: Poverty, malnutrition, poor sanitation create ideal conditions for virus to breed, spread http://www.thestar.com/world/columnist/article/664485 [Accessed 13.12.2009]
  • WHO, 2009, Wild Poliovirus, 31 Dec 2008- 30 June 2009. http://www.ribi.org/assets/_files/images/jul_09/hm__1246721126_casemap.gif [Accessed 18 December 2009]