We are unlikely to ever know all the details of the birth of the AIDS epidemic. But a series of recent genetic discoveries have shed new light on it, starting with the moment when a connection from chimp to human changed the course of history.

We now know where the epidemic began: a small patch of dense forest in southeastern Cameroon. We know when: within a couple of decades on either side of 1900. We have a good idea of how: A hunter caught an infected chimpanzee for food, allowing the virus to pass from the chimp’s blood into the hunter’s body, probably through a cut during butchering.

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