Introduction

The genus Yersinia consists of 17 species. Only three species are consistently pathogenic for humans. These are Yersinia pestis, the agent of plague, Yersinia enterocolitica, and Yersinia pseudotuberculosis, which are usually, but not entirely, gastrointestinal pathogens.

Yersinia pestis

An ongoing result of the unfortunate events of September 11, 2001 has been an increase in research on Yersinia, particularly, Y. pestis. This is an ancient organism and since its divergence from Y. pseudotuberculosis 1500 to 20 000 years ago has largely existed as a rodent pathogen with only occasional human transmission. Three well-described pandemics of plague have occurred. The last of these began in the nineteenth century. Alexandre Yersin first isolated Y. pestis in 1894. The twentieth century saw major outbreaks in Vietnam and India. The majority of cases currently are seen in sub-Saharan Africa, particularly Madagascar.

The majority of cases seen in the United States are from the southwestern states of New Mexico, Arizona, Colorado, Texas, and California. Climate change may well affect the bacteria, the vector, and the hosts, resulting in potential worldwide changes in the frequency and distribution of disease. In 2010 two human cases were reported in Oregon

There has been remarkable progress in the understanding of the pathogenesis of Yersinia infections, particularly those caused by Y. pestis. A partial list of these advances include the effects of Y. pestis on flea behavior, and its avoidance of both innate and adaptive (both humeral and cellular) immune response.