Scanning electron micrograph of Yersinia pestis bacteria on proventricular spines of a Xenopsylla cheopis flea. Yersinia pestis (formerly Pasteurella pestis) is a Gram-negative rod-shaped bacterium. It is a facultative anaerobe that can infect humans and other animals. Human Yersinia pestis infection takes three main forms: pneumonic, septicemic, and the notorious bubonic plagues. All three forms are widely believed to have been responsible for a number of high-mortality epidemics throughout human history, including the Plague of Justinian in 542 and the Black Death that accounted for the death of at least one-third of the European population between 1347 and 1353. More recently, Yersinia pestis has gained attention as a possible biological warfare agent and the CDC has classified it as a category A pathogen requiring preparation for a possible terrorist attack.