Plague Home > What Caused the Plague?

Many people wonder, "What caused the plague?" What caused it then still causes it today -- the Yersinia pestis bacteria. In most cases, the plague is contracted through the bites of an infected rodent or flea. Today, there are only a few thousand cases of plague each year worldwide. Approximately 10 to 20 cases are reported in the Southwest region of the United States.

What Caused the Plague? -- An Overview

Bubonic plague is the most common form of plague. Yersinia pestis are the bacteria that cause it. These bacteria also cause the other two forms of plague: septicemic plague and pneumonic plague (see Types of Plague).

What Caused the Plague? -- Yersinia Pestis

Yersinia pestis are Gram-negative bacteria found in certain parts of the world, causing about 1,000 to 3,000 plague cases each year. Cases of plague most commonly occur in Africa, Asia, and South America. Between 10 and 20 cases of bubonic plague happen in the United States every year, most often in rural areas of the Southwest.
Yersinia pestis is found most commonly in rats, but occasionally in other animals. Other animals known to carry Yersinia pestis include:
  • Mice
  • Fleas
  • Lice
  • Prairie dogs
  • Cats
  • Dogs
  • Squirrels
  • Wood rats
  • Chipmunks.
(Click Bubonic Plague Pictures to see photos of animals that can cause bubonic plague.)

What Caused the Plague? -- Transmission Methods

Usually, bubonic plague is contracted through the bites of an infected flea or rodent. In rare cases, Yersinia pestis bacteria enter through an opening in your skin, either from a piece of contaminated clothing or other material used by an infected person. Bubonic plague is rarely spread from person to person.
Yersinia pestis bacteria travel to the lymph nodes and begin to multiply once they are inside your body. (The lymph or lymphatic system is a major component of your body's immune system. The organs within the lymphatic system are the tonsils, adenoids, spleen, and thymus.)
Within two to six days of exposure to the plague bacteria, bubonic plague symptoms will develop, including:
  • Swollen, tender lymph glands (called buboes, hence the name bubonic)
  • Fever
  • Chills
  • Headache
  • Weakness.
Written by/reviewed by:
Last reviewed by: Arthur Schoenstadt, MD
Last updated/reviewed:
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