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Long before the causes of disease were known and long before the processes of recovery were understood, an interesting thing was observed: if people recovered from a disease, rather than succumbing to it, they appeared to be immune from a second bout with the same illness. Perhaps it was these types of observations that led the Chinese to try to prevent smallpox--a deadly disease characterized by pus-filled blisters--by exposing uninfected individuals to matter from smallpox lesions. This process, known as "variolation," took a variety of forms. One form consisted of removing pus and fluid from a smallpox lesion and using a needle to place it under the skin of the person to be protected. Another method involved peeling scabs from lesions, drying and grinding them to a powder, and letting an uninfected person inhale this powder. The third method involved picking up a small amount of the scab powder with a needle and then using the needle to place the powder directly into the individual's veins. Lady Mary Wortley Montagu, wife of the British Ambassador to Turkey, observed this third method in the early 1700s and brought it back to England. Although the effects of variolation varied, ranging from causing a mild illness in most individuals to causing death in a few, the mortality and morbidity rates due to smallpox were certainly lower in populations that used variolation than in those that did not.
One person who experienced variolation as a child in the late 1700s was Edward Jenner, a young boy who survived the process and grew up to become a country doctor in England. As a country doctor, Jenner noticed a relationship between the equine disease known as "grease" and a bovine disease known as "cow pox." He saw that farmers who treated horses with grease lesions often saw the development of cow pox in their cows, complete with blisters similar to those seen in smallpox infection. Unlike lethal smallpox, however, the cowpox blisters eventually disappeared,...