The genus Yersinia contains three pathogenic species: Y. pestis, Y. enterocolitica and Y. pseudotuberculosis. All pathogenic strains contain a 70 kb plasmid coding for a number of virulence factors, of which outer membrane proteins including an adhesin are the most important. In 65% of the patients the infection is self-limiting, but in the others a chronic local inflammation develops. Persistence of the infection has been associated with the immune status and the HLA-B27 antigen. Y. enterocolitica leads to abdominal complaints in young children that usually subside spontaneously. The infection is more serious with increasing age, and in people older than 30 years may involve a septicaemic form with multiple abscesses in various organs, or a lymphadenopathic form with generalized lymphadenitis mimicking a haematologic malignancy. Yersinia can be easily isolated during the acute phase by culture; during the chronic and persistent phase it is not cultivable anymore. Then serology should be performed, anti-Yop serology being the most reliable. Yersinia is susceptible to most antibiotics, except penicillins. Local penetration of the antibiotic may be a problem, however. First-choice antibiotics are cotrimoxazole, tetracyclines, chloramphenicol and fluoroquinolones. Systemic extra-mesenteric infections should always be treated with antibiotics.