The genus Yersinia comprises eleven species in the family Enterobacteriaceae of the Gammaproteobacteria. Three of them, Y. pestis, Y. pseudotuberculosis, and Yersinia enterocolitica, are important pathogens of humans and other mammals; one, Yersinia ruckeri, is the agent of red mouth disease in rainbow trout and the others are non-pathogens that live in water and soil (Bottone et al. 2005). Y. pestis, the causative agent of plague, differs conspicuously from its fellow Yersinia species. It is much more invasive and virulent than Y. pseudotuberculosis or Y. enterocolitica, which cause relatively benign enteric diseases transmitted via contaminated food and water. With a lethal dose to susceptible mammals from an intradermal inoculation site of less than ten cells, Y. pestis is one of the most virulent of all microbes and plague one of the most feared diseases of human history (Perry and Fetherston 1997). A second, no less remarkable difference is that Y. pestis, uniquely among the enteric group of Gram-negative bacteria, has evolved an arthropod-borne route of transmission. Y. pestis is primarily a parasite of rodents that is transmitted by fleas. Permanent enzootic foci exist throughout the world, and plague transmission cycles involve many species of wild rodents and their fleas, making the ecology and epizootiology of plague quite complex (Gage and Kosoy 2005; Pollitzer 1954).

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