The Mutant Genes Behind the Black Death Only a few genetic changes were enough to turn an ordinary stomach bug into the bacteria responsible for the plague. Pieter Bruegel the ElderThe Triumph of Death (1562), by Pieter Bruegel the Elder. By: Carrie ArnoldOctober 6, 2015 Comments (1) Download PDF Print Each year, 4 million people visit Yosemite National Park in California. Most bring back photos, postcards and an occasional sunburn. But two unlucky visitors this summer got a very different souvenir. They got the plague.This quintessential medieval disease, caused by the bacterium Yersinia pestis and transmitted most often by fleabites,...