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. Y. pestis was discovered in 1894 by Alexandre Yersin, a Swiss/French physician and bacteriologist from the Pasteur Institute, during an epidemic of the plague in Hong Kong. Yersin was a member of the Pasteur school of thought. Kitasato Shibasaburō, a German-trained Japanese bacteriologist who practised Koch's methodology, was also engaged at the time in finding the causative agent of the plague. However, Yersin actually linked plague with Y. pestis. Named Pasteurella pestis in the past, the organism was renamed Yersinia pestis in 1944. Every year, thousands of cases of the plague are still reported to the World Health Organization, although, with proper treatment, the prognosis for victims is now much better. A five- to six-fold increase in cases occurred in Asia during the time of the Vietnam War, possibly due to the disruption of ecosystems and closer proximity between people and animals. Plague also has a detrimental effect on non-human mammals. In the United States, mammals such as the black-tailed prairie dog and the endangered black-footed ferret are under threat.

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