What happened to make plague able to cause devastating epidemics, as in this depiction from 1349? (Credit: Pierart dou Tielt/Wikimedia)

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 dise