An infectious disease, bubonic plague is primarily transmitted through the bites of infected fleas or rodents. The bacteria that cause the disease are rarely spread through human-to-human contact. Symptoms tend to develop within six days. However, other conditions are ruled out before making a firm diagnosis. If left untreated, the mortality rate for bubonic plague is 50 to 90 percent; it drops to 15 percent with early diagnosis and treatment.
Plague is an acute, infectious disease of humans, rodents, and ectoparasites (fleas, lice).
Bubonic plague is 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. (Click Plague and Animals for more information.)
Usually, bubonic plague is spread by being bitten by an infected flea or rodent. In rare cases, Yersinia pestis bacteria that is present on a piece of contaminated clothing or other material used by an infected person may enter through an opening in the skin. The bacteria are rarely spread from person to person.
When a person becomes infected with the bacteria that cause bubonic plague, the bacteria begin to multiply within the lymph system. (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.) After two to six days, symptoms of bubonic plague can begin. The period between becoming infected and the start of symptoms is called the incubation period.