Smallpox is a very contagious and virulent viral disease that has been known since ancient times. Depending on the population exposed to it, mortality ranges from 50% (for those of European descent) to 95% (for those of native American descent). Survivors are often scarred badly by the pustules that give the disease its name. Smallpox is very easy to transmit from person to person and even infected blankets and other bedding can remain virulent for long periods of time. Historically, smallpox has ravaged populations in epidemics. However, it was the first disease for which an effective vaccine was developed and since the 18th century the disease became less common until it was totally eradicated in the 20th century. Smallpox cannot live indefinitely without a human host and has been eradicated in the wild. The only remaining samples of the virus now exist in two labs in Atlanta and Moscow. The last smallpox death was over 20 years ago and was the result of accidental exposure in a laboratory.
Edward Jenner noticed that those who had survived smallpox never contracted the disease again, and milkmaids who had suffered a mild disease known as cowpox also were immune to the disease. In spite of widespread fear and ridicule, Jenner deliberately exposed patients to cowpox and noted that the contagion and mortality rates of such persons decreased to zero, where in a general population over 95% of persons exposed to smallpox would contract it and 50% of a typical population exposed to a case would die of the disease.
Smallpox was eradicated by both a widespread program of vaccination, plus a rapid response to outbreaks where those exposed to the disease were quickly vaccinated against it. Like rabies, the vaccine actually builds antibodies to the illness faster than the actual disease does, so patients can be successfully vaccinated even after they are exposed to the disease.