Yersinia pseudotuberculosis is a very homogeneous species, with only six sero-groups and which can not also be lysotyped. The essential reservoir is made up of rodents, birds, cats and soil and is the origin of human infection. Y. enterocolitica has 5 biochemical types, 57 serotypes (types 03 and 09 are the most commonly involved in human pathology) and 11 lysotypes. Contamination is essentially by direct human-human contact from healthy carriers or, more rarely, via infected food. The aetiological diagnosis of the arthritis occurring after an episode of abdominal Yersinia (enteritis, mesenteric adenitis) depends on the simultaneous search for bacteria in the stools and for antibodies.