Plague is an acute bacterial infection caused by the organism Yersinia pestis. Natural infection occurs in a range of mammalian species including rodents, cats and other carnivores, and humans.
Human plague still occurs in parts of Africa, Asia, South America and rural areas of the USA.
Humans usually acquire the infection through the bite of a flea that has previously fed on an infected animal. (See photo.) This usually produces an ulcer at the site of the bite and enlarged painful lymph nodes (bubonic plague). Pneumonic plague may occur as a complication of bubonic plague or result directly from contact with a person with pneumonic plague.