Yersinia pestis is the causative agent of the various forms of the black plague, including bubonic, septicemic, and pneumonic. It is a gram-negative rod bacterium, and a member of the family Enterobacteriaceae, which includes a wide variety of disease-causing bacteria and three human pathogens. Y. pestis infects a host through the bite of the Oriental Rat Flea. The bacteria multiply until they block the flea's stomach, thus causing the flea to regurgitate bacteria into its host (usually a rat, sometimes a human or another mammal).

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