These Two Mutations Turned Not-so-Deadly Bacteria Into the Plague By Marissa Fressenden Smithsonian.com The bacterium that causes the Black Plague, called Yersinia pestis, has been infecting humans ever since it evolved 5,000 to 10,000 years ago. But its ancestor, Y. pseudotuberculosis, only causes an illness similar to scarlet fever. Most people recover from an Y. pseudotuberculosis infection after a few weeks. The Plague, of course, is […] Source: VT