In an epidemic those people with genes for resistance to that particular microbe are more likely to survive than are people lacking such genes. As a result, over the course of history, human populations repeatedly exposed to a particular pathogen have come to consist of a higher proportion of individuals with those genes for resistance-just because unfortunate individuals without the genes were less likely to survive to pass their genes on to babies.
( Jared Diamond )
[ Guns, Germs, and Steel ]
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