Yersinia, (genus Yersinia), any of a group of ovoid- or rod-shaped bacteria of the family Enterobacteriaceae. Yersinia are gram-negative bacteria and are described as facultative anaerobes, which means that they are capable of surviving in both aerobic and anaerobic environments. Though several species are motile below 37 °C (98.6 °F), all Yersinia organisms are rendered nonmotile at this temperature and above. The genus is named for French bacteriologist Alexandre Yersin, who in 1894 discovered Pasteurella pestis (now Yersinia pestis), the causative agent of plague, which was independently isolated that same year by Japanese physician and bacteriologist Kitasato Shibasaburo.

In addition to Y. pestis, other species that are important pathogens in humans include Y. enterocolitica and Y. pseudotuberculosis. Y. enterocolitica is widespread in domestic animals, including pigs and cattle, and is found in birds and in aquatic species, such as frogs and oysters. It also has been isolated from soil and from the surface layers of various bodies of water, including lakes and streams; its entry into soil and water systems originates with animal wastes. The organism is transmitted to humans as a foodborne or waterborne pathogen, and infection results in an acute gastrointestinal condition known as yersiniosis. A similar condition arises following infection with Y. pseudotuberculosis; however, little is known about its mode of transmission to humans. Y. pseudotuberculosis appears to circulate in a variety of animals and has been found in horses, cattle, dogs, cats, rabbits, rodents, deer, and birds, including ducks, geese, turkeys, and canaries. In some instances, infection with either Y. pseudotuberculosis or Y. enterocolitica may give rise to mesenteric lymphadenitis, an inflammation of the peritoneal tissue of the intestines that produces symptoms similar to those of appendicitis. In contrast to the other Yersinia organisms, Y. pestis circulates in rodents and is transmitted to humans through the bite of an infected flea.

Several other Yersinia organisms have been identified, including Y. intermedia, Y. frederiksenii, and Y. ruckeri. The latter is pathogenic in salmonids (family Salmonidae), including rainbow trout and Pacific salmon. In these species, Y. ruckeri causes enteric redmouth disease, which is characterized by hemorrhaging of the subcutaneous tissues under the fins and around the eyes and mouth.