Eradication means a disease has been removed from the whole world, so there are no more cases anywhere and control measures such as vaccination can stop. This is different from elimination, where a disease is removed from one country or region but still exists elsewhere.Smallpox is the only human disease to have been eradicated. The World Health Organization ran a global vaccination campaign, the last natural case was in 1977 and smallpox was declared eradicated in 1980. Rinderpest, a disease of cattle, was declared eradicated in 2011.Smallpox was a good target for several reasons:
• the virus showed very little antigenic variability, so one vaccine worked everywhere and for life
• there was no animal reservoir: the virus only infected humans
• infected people had an obvious rash, so cases were easy to identify
• the vaccine was a live virus that could be freeze-dried, so it stayed effective in hot countries without refrigeration
• ring vaccination was used: when a case appeared, everyone around the patient was vaccinated, containing the outbreak.Many other diseases are much harder to eradicate. Influenza and HIV show high antigenic variability, so vaccines quickly become ineffective. Malaria is caused by a protoctist that hides inside body cells and changes its antigens at different stages of its life cycle. Diseases with animal reservoirs, such as yellow fever, or with symptomless carriers, keep re-infecting people. War, poverty and poor infrastructure also make it difficult to vaccinate enough people.Polio is close to eradication: wild poliovirus now survives in only a very small number of countries.
• the virus showed very little antigenic variability, so one vaccine worked everywhere and for life
• there was no animal reservoir: the virus only infected humans
• infected people had an obvious rash, so cases were easy to identify
• the vaccine was a live virus that could be freeze-dried, so it stayed effective in hot countries without refrigeration
• ring vaccination was used: when a case appeared, everyone around the patient was vaccinated, containing the outbreak.Many other diseases are much harder to eradicate. Influenza and HIV show high antigenic variability, so vaccines quickly become ineffective. Malaria is caused by a protoctist that hides inside body cells and changes its antigens at different stages of its life cycle. Diseases with animal reservoirs, such as yellow fever, or with symptomless carriers, keep re-infecting people. War, poverty and poor infrastructure also make it difficult to vaccinate enough people.Polio is close to eradication: wild poliovirus now survives in only a very small number of countries.