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New diseases have struck human populations since humans first evolved (and before that, they struck earlier primates). Generally they follow a predictable path: at first, they kill a lot of people. If they kill too many too quickly, there’s nobody left to pass the disease along, and it dies out. If they are less serious, they can infect more people. So diseases tend to evolve to be less dangerous, especially to children. Many diseases that were probably once very serious are now viruses that most of us catch as small children, that we hardly notice, like roseola. Others are on their way there, like hand foot and mouth disease and chickenpox. We think of measles as a childhood disease, but it, too, used to be much more serious. The plague Thucydides describes in Athens may have been the first attack of measles (but it might also have been a different disease, maybe one that died out so we don’t know it now.)

This Athenian plague spread easily through Athens because people were living crowded together, and many of them were unhoused, living in shacks or tents, or just sleeping outside. That was because the war had made it dangerous to live outside the city walls, so everyone had moved inside the city. The best Greek general, Pericles, died of the plague, along with thousands of other people.

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