Plague is an infectious disease that is caused by the bacterium Yersinia pestis. Depending on lung infection, or sanitary conditions, plague can be spread in the air, by direct contact, or very rarely by contaminated undercooked food. The symptoms of plague depend on the concentrated areas of infection in each person: bubonic plague in lymph nodes, septicemic plague in blood vessels, pneumonic plague in lungs. It is treatable if detected early. Plague is still relatively common in some remote parts of the world. Until June 2007, plague was one of the three epidemic diseases specifically reportable to the World Health Organization (cholera and yellow fever the other two). The bacterium is named after the French-Swiss bacteriologist Alexandre Yersin. Historically, what are thought to have been massive pandemics of plague swept through Eurasia with very high death rates and causing major cultural changes. The largest of these were the Plague of Justinian of 541–542, The Black Death of the 1340s, continuing in the Second plague pandemic to break out at intervals, and the Third plague pandemic beginning in 1855 and considered inactive from 1959. The epidemiological use of the term plague is currently applied to any severe bubo inflammation resulting from an infection with Y. pestis. Historically, the medical use of the term plague has been applied to pandemic infections in general. Plague is often synonymous with bubonic plague, but this describes just one of its manifestations. Other names have been used to describe this disease, such as Black Plague and the Black Death; the latter is now used primarily by scholars to describe the second, and most devastating, pandemic of the disease. The etymology of the word plague is believed to come from the Latin word plāga ("blow, wound") and plangere (“to strike, or to strike down”), cf. German Plage (“infestation”).

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