“Epidemics are, by their very nature, divisive. The neighbor you might, in better times, turn to for help becomes a possible source of infection. The rituals of daily life become opportunities for transmission; the authorities enforcing quarantine become agents of oppression. Time and time again throughout history, people have blamed outsiders for outbreaks. ... [Author Frank M.] Snowden recounts the story of what happened to the Jews of Strasbourg during the Black Death. Local officials decided that they were responsible for the pestilence—they had, it was said, poisoned the wells—and offered them a choice: convert or die. Half opted for the former. On February 14, 1349, the rest ‘were rounded up, taken to the Jewish cemetery, and burned alive.’ Pope Clement VI issued papal bulls pointing out that Jews, too, were dying from the plague, and that it wouldn't make sense for them to poison themselves, but this doesn't seem to have made much difference. In 1349, Jewish communities in Frankfurt, Mainz, and Cologne were wiped out. To escape the violence, Jews migrated en masse to Poland and Russia, permanently altering the demography of Europe.”

— Elizabeth Kolbert, “Pandemics and the Shape of Human History,” The New Yorker, March 30, 2010. Other possible historical shapes: ending the Roman Empire; killing off New World civilizations; creating a need for the African slave trade; setting the stage for the Russian Revolution.