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Diseases have evolved by making humans sick in ways that allow the diseases to spread without killing the human host too quickly. Both of these are important for germs.
Germs need to make people sick so that the germs can spread more easily. They need people to expel germs by vomiting or sneezing or having diarrhea. The germs evolved in order to cause human beings to do these things. At the same time, germs need their hosts not to die quickly, before the germ can spread. The germ does not benefit from the host's death. For this reason, Diamond says, germs evolved to cause diseases that are serious and which recur in a human population but do not kill all the people.
By evolving in these ways, germs came to cause epidemic diseases that stayed latent in a human population like those in the Old World.
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