One of civilization’s most prolific killers shadowed humans for thousands of years without their knowledge. The bacteria Yersinia pestis, which causes the plague, is thought to be responsible for up to 200 million deaths across human history — more than twice the casualties of World War II. The Y. pestis death toll comes from three widespread disease outbreaks, known as epidemics: the sixth century Justinianic Plague that ravaged the Eastern Roman Empire; the 14th century Black Death that killed somewhere between 40 percent and 60 percent of the European population; and the ongoing Third Pandemic, which began in China in…

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