Introductionplague, any contagious, malignant, epidemic disease, in particular the bubonic plague and the black plague (or Black Death), both forms of the same infection. These acute febrile diseases are caused by Yersinia pestis ( Pasteurella pestis ), discovered independently by Shibasaburo Kitasato and Alexandre Yersin in 1894, a bacterium that is transmitted to people by fleas from rats, in which epidemic waves of infection always precede great epidemics in human populations. Sylvatic plague, still another form, is carried by other rodents, e.g., squirrels, rabbits, chipmunks, in rural or wooded areas where they are prevalent.
Bubonic plague, the most common form, is characterized by very high fever, chills, prostration, delirium, hemorrhaging of the small capillaries under the skin, and enlarged, painful lymph nodes (buboes), which suppurate and may discharge. Invasion of the lungs by the organism (pneumonic plague) may occur as a complication of the bubonic form or as a primary infection. Pneumonic plague is rapidly fatal and is the only type that can be spread from person to person (by droplet spray) without intermediary transmission by flea. In the black form of plague, hemorrhages turn black, giving the term "Black Death" to the disease. An overwhelming infection of the blood may cause death in three or four days, even before other symptoms appear.
In untreated cases of bubonic plague the mortality rate is approximately 50%–60%; pneumonic plague is usually fatal if not treated within 24 hours. Such antibiotics as streptomycin and tetracycline greatly reduce the mortality rate. Vaccine is available for preventive purposes. Rodent control is important in areas of known infection.
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