DefinitionPlague is a severe and potentially deadly bacterial infection.Alternative NamesBubonic Plague; Pneumonic plague; Septicemic plagueCauses, incidence, and risk factorsPlague is caused by the bacteria Yersinia pestis.Rodents, such as rats, carry the disease. It is spread by their fleas.People can get the plague when they are bitten by a flea that carries the plague bacteria from an infected rodent. In rare cases, you may get the disease when handling an infected animal.A plague lung infection called pneumonic plague can spread from human to human. When someone with pneumonic plague coughs, tiny droplets carrying the bacteria move through the air. Anyone who breathes in these particles may catch the disease. An epidemic may be started this way.In the Middle Ages in Europe, massive pl