TheYersiniagenus includesYersinia enterocolitica, Yersinia pseudotuberculosis, Yersinia pestis, Yersinia frederiksenii, Yersinia kristensenii, Yersinia intermedia, Yersinia aldovae, andYersinia rohdei. The first three species are pathogenic for immunocompetent humans.Y. enterocoliticahas emerged as an increasing worldwide pathogen. Recent studies have elucidated new data on its epidemiology, the pathogenic mechanisms of invasion, and new clinical problems.Y. pseudotuberculosisis a much less common pathogen that produces gastrointestinal manifestations similar toY. enterocolitica, and also on rare occasions sepsis. It will not be discussed further in this review.Y. pestisis the etiologic agent of plague. This pathogen produces one of the classic infectious diseases with enormous historic implications and is still a significant problem in the United States and many parts of the third world.