Bubonic plague is the most common form of plague, an illness that occurs in humans, rodents, and ectoparasites (fleas and lice). The condition is caused by Yersinia pestis bacteria and characterized by large, infected lymph nodes called buboes.

 

When an animal or person is infected with Yersinia pestis, the bacteria affect the lymph system, causing the characteristic symptoms. If bubonic plague is left untreated, plague bacteria can quickly multiply in the bloodstream, causing septicemic plague. The bacteria could also progress to the lungs, causing pneumonic plague. The mortality rate of bubonic plague is 50 to 90 percent if not treated; the mortality rate is 15 percent when this condition is diagnosed and treated early.