Plague is an infectious disease caused by Yersinia pestis bacteria, usually found in small mammals and their fleas. The disease is transmitted between animals via their fleas and, as it is a zoonotic bacterium, it can also transmit from animals to humans.

    Humans can be contaminated by the bite of infected fleas, through direct contact with infected materials, or by inhalation. Plague can be a very severe disease in people, particularly in its septicaemic and pneumonic forms, with a case-fatality ratio of 30% - 100% if left untreated.

    Although plague has been responsible for widespread pandemics throughout history, including the so-called Black Death that caused over 50 million deaths in Europe during the fourteenth century, today it can be easily treated with antibiotics and the use of standard preventative measures.

    Plague is found on all continents except Oceania but most human cases since the 1990s have occurred in Africa. Democratic Republic of Congo, Madagascar and Peru are the three most endemic countries.


    People infected with plague usually develop influenza-like symptoms after an incubation period of 3–7 days. Symptoms include fever, chills, aches, weakness, vomiting and nausea.

    There are 3 main forms of plague.

    Bubonic plague is the most common and is caused by the bite of an infected flea. The plague bacillus, Y. pestis, enters at the bite and travels to the nearest lymph node to replicate. The lymph node becomes inflamed, tense and painful, and is called a bubo. With advanced infections, the inflamed lymph nodes can turn into suppurating open sores. Bubonic plague cannot be transmitted from human to human.

    Septicaemic plague occurs when infection spreads through the bloodstream. It may result from flea bites or 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, it 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 to other humans. This is usually fatal.


    Untreated plague can be rapidly fatal, so early diagnosis and treatment is essential for survival and to reduce complications. Antibiotics and supportive therapy are effective against plague if patients are diagnosed in time.

    Laboratory testing is required to confirm infection. This includes identifying the causative bacteria in a sample of pus from a bubo, blood or sputum. WHO does not recommend vaccination except for high-risk people such as laboratory personnel and health care workers.

    Preventive measures include informing people when zoonotic plague is present in their environment and giving advice on how they can protect themselves. They should be advised to take precautions against flea bites and not to handle animal carcasses. People, especially health workers, should also avoid direct contact with infected tissues such as buboes, or close exposure to patients with pneumonic plague.

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