Plague is an infectious disease caused by the bacteria Yersinia Pestis, a zoonotic bacteria, usually found in small mammals and their fleas. It is transmitted between animals and humans by the bite of infected fleas, direct contact, inhalation and rarely, ingestion of infective materials.

Plague can be a very severe disease in people, with a case-fatality ratio of 30%-60% if left untreated. In 2015 there were 320 cases reported worldwide, including 77 deaths. Historically, plague was responsible for widespread pandemics with high mortality. It was known as the "Black Death" during the fourteenth century, causing an estimated 50 million deaths, approximately half of them in Asia and Africa and the other half in Europe, where a quarter of the population succumbed.

People infected with plague usually develop “flu-like” symptoms after an incubation period of 3-7 days. Typical symptoms are the sudden onset of fever, chills, head and body-aches and weakness, vomiting and nausea.

There are 3 main forms of plague infection, depending on the route of infection: bubonic, septicaemic and pneumonic.

Bubonic plague is the most common form of plague and is caused by the bite of an infected flea. Plague bacillus, Y. pestis, enters at the bite and travels through the lymphatic system to the nearest lymph node where it replicates itself. The lymph node then becomes inflamed, tense and painful, and is called a "bubo". At advanced stages of the infection the inflamed lymph nodes can turn into suppurating open sores. There is no inter human transmission of bubonic plague.

Septicaemic plague occurs when infection spreads through the bloodstream Septicaemic plague may result from flea bites and from direct contact with infective materials through cracks in the skin. Advanced stages of the bubonic form of plague will also lead to direct spread of Y. pestis in the blood.

Pneumonic plague-or lung-based plague- is the most virulent and least common form of plague. Typically, the pneumonic form is caused by spread to the lungs from advanced bubonic plague. However, a person with secondary pneumonic plague may form aerosolized infective droplets and transmit plague via droplets to other humans. Untreated pneumonic plague has a case-fatality ratio close to 100%.

Diagnosis and confirmation of plague requires laboratory testing. The best way to confirm that a patient has plague is to identify Y Pestis in a sample of fluid from a bubo, or blood or sputum. Rapid dipstick tests have been validated for field use to quickly screen for Y. pestis antigen in patients. Specimens should be collected and safely forwarded to laboratories for plague testing.

Untreated plague can be rapidly fatal so early diagnosis and treatment is essential for survival and reduction of complications. Antibiotics and supportive therapy are effective against plague if patients are diagnosed in time. Pneumonic plague patients must be isolated and cared by trained medical staff with personal protective equipment.

Preventive measures include informing people when zoonotic plague is active in their environment and advising them to take precautions against flea bites and not to handle animal carcasses in plague-endemic areas. In plague endemic areas and during bubonic plague outbreaks, flea and reservoir (usually rodents) controls must be implemented.

Pneumonic plague is highly contagious. Close contact persons must be kept under medical surveillance and must receive a prophylaxis with antibiotics during 7 days.

Plague vaccines were widely used before the antibiotics era. The current vaccines have not been shown to be very effective against pneumonic plague and are not recommended by WHO out of for high-risk groups (e.g. laboratory personnel who are constantly exposed to the risk of contamination).