The English physician Edward Jenner found that if experimental subjects were deliberately infected with cowpox, which caused only a mild illness, they are immune from smallpox.
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Awhich caused only a mild illness, they are immune from
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Bcausing only a mild illness, they become immune from
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Cwhich causes only a mild illness, they are immune to
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Dcausing only a mild illness, they became immune from
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Ewhich caused only a mild illness, they would become immune to
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