Humans do not usually catch infectious diseases from animals; pathogens tend to confine their nasty work to a single species or genus. (Leishmaniasis is a striking exception.) But microbes mutate all the time. Once in a while, an animal pathogen will change in such a way that it suddenly infects a person. When people in the Near East first domesticated cattle from a type of wild ox called an aurochs, a mutation in the cowpox virus allowed it to jump into humans—and smallpox was born. Rinderpest in cattle migrated to people and became measles. Tuberculosis probably originated in cattle,
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