The deadliest and most devastat-ing opponents our species has faced have never been across a battlefield. They've been on our skin and in our blood and bones. Viruses and bacteria have killed or debilitated millions during the course of human history - and well before. Researchers have unraveled the story of a few, most notably the bacterium Yersinia pestis, which causes plague. But findings about the origins and evolution of other scourges, including leprosy, hepatitis B and syphilis, have been elusive or contradictory.
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