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Plague is an infectious disease caused by bacteria called Yersinia pestis. These bacteria are found mainly in rodents, particularly rats, and in the fleas that feed on them. Other animals and humans usually contract the bacteria from rodent or flea bites.

white-throated wood rat
A white-throated woodrat, Neotoma albigula, in its Southwestern U.S. arid habitat. This, and other woodrat species, harbor fleas known to carry Yersinia pestis bacteria, which may be transmitted to human beings through flea bites.
Credit: CDC

Historically, plague destroyed entire civilizations. In the 1300s, the "Black Death," as it was called, killed approximately one-third (20 to 30 million) of Europe's population. In the mid-1800s, it killed 12 million people in China. Today, thanks to better living conditions, antibiotics, and improved sanitation, current World Health Organization statistics show there were only 783 cases in 2013 worldwide.

In recent decades, an average of seven people per year in the United States develop plague, according to the Centers for Disease Control and Prevention. There has not been a case of person-to-person infection in the United States since 1924.



Yersinia pestis bacteria are found in animals throughout the world, most commonly in rats but occasionally in other wild animals, such as prairie dogs. Most cases of human plague are caused by bites of infected animals or the infected fleas that feed on them. In almost all cases, only the pneumonic form of plague can be passed from person to person.​


A healthcare provider can diagnose plague by doing laboratory tests on blood or sputum, or on fluid from a lymph node, from an infected person.


If a healthcare provider suspects plague and diagnoses it early, they can prescribe specific antibiotics.

Untreated bubonic plague bacteria can quickly multiply in the bloodstream causing septicemic plague, or even move to the lungs, causing pneumonic plague.



Health experts recommend antibiotics if you have been exposed to wild rodent fleas, during a plague outbreak in animals, or to a possible plague-infected animal. Because there are so few cases of plague in the United States, experts do not recommend taking antibiotics unless it's certain a person has been exposed to plague-infected fleas or animals.


Currently, there is no commercially available vaccine against plague in the United States.

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Last Updated January 06, 2015