A genus of bacteria in the Enterobacteriaceae family. The bacteria appear as gram-negative rods and share many physiological properties with related Escherichia coli. Of the 11 species of Yersinia, Y. pestis, Y. enterocolitica, and Y. pseudotuberculosis are etiological agents of human disease. Yersinia pestis causes flea-borne bubonic plague (the black death), an extraordinarily acute process believed to have killed over 200 million people during human history. Enteropathogenic Y. pseudotuberculosis and Y. enterocolitica typically cause mild chronic enteric infections. The remaining species either promote primary infection of fish (Y. ruckeri) or exist as secondary invaders or inhabitants of natural environments (Y. aldovae, Y. bercovieri, Y. frederiksenii, Y. intermedia, Y. kristensenii, Y. mollaretii, and Y. rohdei).