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Observation and deductive reasoning. Smallpox was a scourge during Jenner's time, but he noticed that milkmaids contracted a very mild illness similar to smallpox called "cowpox", from which they recovered easily. He postulated that a tiny bit of the cowpox serum could be injected into a well person, and the mild cowpox might protect them from the deadly smallpox. It worked.

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14y ago

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