Yersinia pestis (formerly Pasteurella pestis) is a Gram-negative rod-shaped bacterium belonging to the family Enterobacteriaceae. It is a facultative anaerobe that can infect humans and other animals. Human Y. pestis infection takes three main forms: pneumonic, septicemic, and the notorious bubonic plagues. All three forms have been responsible for high mortality rates in epidemics throughout human history, including the Black Death (a bubonic plague) that accounted for the death of at least one-third of the European population in 1347 to 1353.