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The plague

Types of Plague


There are three different types of plague: bubonic plague, septicemic plague, and pneumonic plague. The most common type is bubonic plague. Septicemic plague occurs when the Yersinia pestis bacteria (the organism responsible for the disease) multiply in the blood. Pneumonic plague is the most serious of the three types of plague. It occurs when plague bacteria infect the lungs, causing pneumonia.

prairie dog
A prairie dog in Gunnison, Colo., eats bait laden with plague vaccine. (U.S. Geological Survey)

Key facts

  • Plague is caused by the bacteria Yersinia pestis, a zoonotic bacteria, usually found in small mammals and their fleas.
  • People infected with plague usually develop “flu-like” symptoms after an incubation period of 1-7 days.
  • There are 2 main clinical forms of plague infection: bubonic and pneumonic. Bubonic is the most common form and is characterized by painful swollen lymph nodes or ‘buboes’.
  • Plague can be a very severe disease in people, with a case-fatality ratio of 30%-60% for the bubonic type and is always fatal for the pneumonic kind, if left untreated.
  • 2010 – 2015 there were 3248 cases reported worldwide, including 584 deaths.
  • Currently, the 3 most endemic countries are Madagascar, the Democratic Republic of the Congo and Peru.

Signs and symptoms

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 forms of plague infection, depending on the route of infection: bubonic, septicaemic and pneumonic.

  • Bubonic plague (known in mediaeval Europe as the ‘Black Death’) 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 open sores filled with puss.
  • Pneumonic plague-or lung-based plague- is the most virulent form of plague. Incubation period can be as short as 24 hours. 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 is always fatal.
  • Septicaemic plague occurs when infection spreads through the bloodstream, following a bubonic or a pneumonic plague.

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