Introduction and Objective:
Pathogenic species of bacteria belonging to the Yersinia genus are prevalent in the environment. Among three pathogenic species occurring in Poland, the main species is Y. enterocolitica, which is responsible for the inflammation of the intestines and mesenteric lymph nodes. Although most of the cases are mild, self­limited, internal organs such as the liver, spleen, joints and heart may sometimes be affected. In immunocompromised people, the disease may even be fatal.

Brief description of the state of knowledge:
The source of human infection is meat from infected animals, raw milk, plants and water contaminated with the faeces of infected animals. Y. enterocolitica is a cold­loving bacterium, so it can multiply under refrigeration conditions, and infections occur mostly in the colder months. The symptoms of yersiniosis are non­specific; therefore, the disease is often undiagnosed. The species Y. pseudotuberculosis is much less common. The third species, Y. pestis, is the causative agent of plague which has killed millions of people in the past. However, at present, plague continues to take victims, but to a much lesser extent (1,000–3,000 a year). It occurs outside Europe in rural areas with low sanitary level. Genetic studies revealed that the species Y. pestis separated from the species Y. pseudotuberculosis only several thousand years ago, while Y. pseudotuberculosis and Y. enterocolitica diverged millions of years ago.

Summary:
The pathomechanisms responsible for the virulence of Yersinia bacteria accompanying infections were discussed, and a successful attempt at experimental survival of Yersinia pestis in soil was signalled. The current epidemiological situation worldwide, and the methods of laboratory diagnosis of plague and its prevention were presented.