Yersinia pestis (formerly Pasteurella pestis) is a Gram-negative, rod-shaped coccobacillus, a facultative anaerobic organism that can infect humans via the oriental rat flea. It causes the deadly disease called Bubonic Plague (or "the Plague" colloquially).Human Y. pestis infection takes three main forms: pneumonic, septicemic, and bubonic plagues. All three forms were responsible for a number of high-mortality epidemics throughout human history, including: the sixth century's Plague of Justinian; the Black Death, which accounted for the death of at least one-third of the European population between 1347 and 1353; and the 19th century's Third Pandemic. These plagues probably originated in China and were transmitted west via trade routes.