Plague is a terrifying mystery.
In the Middle Ages, it wiped out 40 million people -- 40 percent of the total population in Europe. Seven hundred years earlier, the Justinian Plague destroyed the Byzantine Empire and ushered in the Middle Ages. The plague of London in the seventeenth century killed more than 1,000 people a day. In the early twentieth century, plague again swept Asia, taking the lives of 12 million in India alone.
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