As a youth, Edward Jenner noticed that people who caught the relatively harmless disease cowpox did not contract smallpox. In 1796 he inoculated a young boy with matter taken from a dairymaid's fresh cowpox lesions. The boy caught cowpox and, when subsequently inoculated with smallpox, did not contract the deadlier disease. The procedure spread and the death rate from smallpox fell.
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