Plague is an infectious disease caused by the bacterium Yersinia pestis. Symptoms include fever, weakness, and headache. Usually this begins one to seven days after exposure. In the bubonic form there is also swelling of lymph nodes, while in the septicemic form tissues may turn black and die, and in the pneumonic form shortness of breath, cough, and chest pain may occur.

Bubonic and septicemic plague is generally spread by flea bites or handling an infected animal. The pneumonitic form is generally spread between people through the air via infectious droplets. Diagnosis is typically by finding the bacterium in fluid from a lymph node, blood, or sputum.

Those at high risk may be vaccinated. Those exposed to a case of pneumonic plague may be treated with preventative medication. If infected, treatment is with antibiotics and supportive care. Typically antibiotics include a combination of gentamicin and a fluoroquinolone. The risk of death with treatment is about 10% while without it is about 70%.

Globally about 600 cases are reported a year. In 2017 the countries with the most cases include the Democratic Republic of the Congo, Madagascar, and Peru. In the United States infections usually occur in rural areas. It has historically occurred in large outbreaks, with the most well known being the Black Death in the 14th century which resulted in greater than 50 million dead.