The worst were plagues that changed the course of history and spelled doom for societies. They even changed human evolution. Survivors of plagues were the genetically lucky ones who had inherited a resistance to the disease-causing organisms. Even in the most extreme plagues, there are resistant people who either do not become infected, no matter how many times they are exposed to the sickness, or who get only a mild disease and recover. When everyone else is dying, the resistant people will be the ones who remain to propagate. Their genes will begin to predominate. And those who were
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