"Plague" is a term used to describe an infectious disease that affects humans and some animals. The Yersinia pestis bacteria are responsible for it, and actually cause three types of the disease: bubonic, septicemic, and pneumonic. If left untreated, the mortality rate is 50 to 90 percent. If treated early (typically with antibiotics), the rate drops to 15 percent.
Plague is an acute, infectious disease of humans, rodents, and ectoparasites (fleas and lice).
Throughout history, this disease has destroyed entire civilizations. In the 1300s, the "Black Death," as it was called, killed approximately one-third (20 to 30 million) of Europe's population. In the mid-1800s, it killed 12 million people in China. Thanks to better living conditions, antibiotics, and improved sanitation, it is rare these days, occurring in a few thousand people worldwide each year.
(Click History of Plague for more information.)
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.
A Yersinia pestis infection can cause one of three forms of plague:
Depending on the circumstances, these different types may occur separately or in combination.
(Click Types of Plague for more information.)