Yersinia pestis is the Gram-negative bacterium responsible for plague, one of history's most devastating infectious diseases in three major pandemic forms: bubonic, septicemic, and pneumonic plague. It is primarily a pathogen of rodents and other small mammals, transmitted to humans via flea bites or direct contact, and is found in natural foci (plague reservoirs) across Asia, Africa, and the Americas. Human cases remain sporadically reported from endemic regions, and the organism is classified as a potential bioterrorism agent.