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Lyme Disease

The Natural History of Disease


Plague is an extremely infectious disease caused by the bacteria Yersinia pestis. It is transmitted between rodents by flea bites or ingestion of the feces of fleas. Fleas are blood-feeding insects (ectoparasites) of birds and mammals. Unlike mosquitoes, both the male and female fleas feed on blood. Blood supplies the "yoke" protein for egg development. Pain and itching results from an allergic reaction to the materials that the fleas inject into the skin at the time of the bite.

In San Diego County, from 1991-1998, ground squirrels were found to be plague-positive in six campgrounds located at higher elevations. Posted signs warn against feeding and handling the squirrels. Vector Surveillance Control does flea control in the campgrounds. Ground squirrels are a nuisance in campgrounds because people enjoy feeding them, and they enjoy the handouts. However, it is unwise to feed wild animals or to leave any garbage behind, assuming that a wild animal will want to eat it, for human food could make them sick. When wild animals are "tamed" it is difficult to determine if their begging and sometimes aggressive behavior means they are sick or merely insisting on being fed. Plague is endemic in California, Utah, Arizona, Nevada, New Mexico, and Texas. Human cases are rare but can happen.


  • Bubonic plague symptoms include sudden onset of high fever; chills; general discomfort; uneasiness or ill feeling (malaise); muscular pains; severe headache; smooth, oval, reddened, painful swellings called buboes in the groin, armpits, neck, or elsewhere in the body; swollen lymph glands; and seizures.
  • Pneumonic plague symptoms include cough, frothy and bloody sputum, and difficulty in breathing.


Immediate treatment with antibiotics is important. Oxygen, intravenous fluids, and respiratory support are additional treatments. Strict isolation is important, and anyone who has had contact with an infected person must be watched closely. If not treated, half the bubonic plague victims die and almost all the pneumonic plague victims die. Treatment lowers the death rate to 5%.

Inform yourself

Fleas—San Diego County Department of Environmental Health
Plague: Introduction—Centers for Disease Control
Plague— Medical Encyclopedia.

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