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Causes
By Mayo Clinic staff
Plague has afflicted humans throughout history. The cause of plague, the Yersinia pestis bacterium, was discovered in 1894 by Alexandre Yersin. Soon after, scientists realized that fleas transmitted the bacteria. Three types of plague exist.
Bubonic plague
Bubonic plague is the most common type of plague in humans. It's usually caused by a bite from an infected flea. Y. pestis bacteria primarily infect animals such as squirrels, rabbits and prairie dogs. You may become infected by a fleabite if you're in close contact with such animals. The bacteria can also enter through a cut in your skin if you handle these animals. Domestic cats that come into contact with infected animals also may transmit the infection to humans.
Septicemic plague
Septicemic plague occurs when plague bacteria multiply in your bloodstream. This happens when bacteria transmitted by a fleabite enter directly into your bloodstream, or as a complication of bubonic or pneumonic plague.
Pneumonic plague
Secondary pneumonic plague can develop if you're infected with another type of plague. In this case, the infection spreads to your lungs, causing a virulent pneumonia that can often be fatal. Primary pneumonic plague can occur when you inhale droplets coughed into the air by a person or animal with pneumonic plague.
Plague as a bioterrorism agent
Plague is also one of a number of potential agents of bioterrorism, along with anthrax, smallpox, botulism, tularemia and nerve gases. It's possible that plague bacteria could be turned into an aerosol and then be spread over large populations.
