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In 1775 Jenner began to study the relationship between cowpox (a comparatively harmless disease) and the more dangerous and disfiguring smallpox. He observed that dairy maids, many of whom had contracted cowpox in their exposure to cows, rarely came down with smallpox. He hypothesized that exposure to cowpox rendered the body immune from smallpox. After nearly twenty years of experiments, he developed the first vaccine. He confirmed his work in 1796, when he inoculated a boy with cowpox and then exposed him to smallpox several weeks later. He published his results in An Inquiry into the Causes and Effects of the Variolae Vaccinae (1798).
Widespread immunization followed, at first for smallpox, but later for other diseases. By the 1970s, smallpox was apparently entirely eliminated from the earth.