Yersinia is a genus of bacteria in the family Yersiniaceae. Its species are Gram-negative, coccobacilli bacteria, a few micrometers long and fractions of a micrometer in diameter, and are facultative anaerobic. Some members of Yersinia are pathogenic in humans, such as Yersinia enterocolitica, Yersinia pseudotuberculosis and Yersinia pestis. Rodents are the natural reservoirs of Yersinia, less frequently, other mammals serve as hosts. Yersinia species are further found in soil and waters.

Infection may occur either through blood (in the case of Y. pestis) or in an alimentary fashion, occasionally via consumption of food products (especially vegetables, milk-derived products, and meat) contaminated with infected urine or feces.