is an infectious disease of animals and humans caused by a bacterium named Yersinia pestis. People usually get plague from being bitten by a rodent flea that is carrying the plague bacterium or by handling an infected animal. Millions of people in Europe died from plague in the Middle Ages, when homes and places of work were inhabited by flea-infested rats.

In the United States, the last urban plague epidemic occurred in Los Angeles in 1924-25. Since then, human plague in the United States has occurred as mostly scattered cases in rural areas (an average of 10 to 15 persons each year). Globally, the World Health Organization reports 1,000 to 3,000 cases of plague every year.

Transmission Cycle:
Fleas usually become infected by feeding on rats and certain other rodents that are circulating the bacterium Yersinia pestis in their bloodstreams. Fleas transmit the plague bacteria to humans and other mammals while feeding on the blood of these animals. When the other person has plague pneumonia and cough, droplets containing plague bacteria can be spread into the air. If a non-infected person inhales these infectious droplets, they also can become infected.

Signs and Symptoms:
The typical sign of the most common form of human plague is a swollen and very tender lymph gland, accompanied by pain. The swollen gland is called a “bubo” (hence the term “bubonic plague”). Buboes usually appears in the groin, armpit or neck region. Bubonic plague should be suspected when a person develops a swollen gland, fever, chills, headache, and extreme exhaustion, and has a history of possible exposure to infected animals or fleas. Pneumonic plague, a less common but much more dangerous form of the disease, is characterized by high fever, cough, bloody sputum and difficulty in breathing.

A person usually becomes ill with bubonic plague 2 to 6 days after being infected. When bubonic plague is left untreated, plague bacteria invade the bloodstream. When plague bacteria escape the person’s immune defenses and freely multiply in the bloodstream, they can spread rapidly throughout the body and cause a severe and often fatal condition called septicemic plague. Infection of the lungs with the plague bacterium causes the pneumonic form of plague, a severe respiratory illness. The infected person may Plague experience high fever, chills, cough with bloody sputum and breathing difficulties. Although pneumonic plague may occur secondarily as a consequence of untreated bubonic or septicemic plague, it also can result from inhaling infectious respiratory droplets or other materials. The incubation period for pneumonic plague cases acquired by inhalation is usually about 2 days.

If plague patients are not given specific antibiotic therapy, the disease can progress rapidly to death.

Mortality Rate:
About 14% (1 in 7) of all plague cases in the United States are fatal. Most cases in the U.S. receive some antibiotic treatment during their course of illness and deaths typically result from delays in seeking treatment or misdiagnosis. Reportedly, about 50-60% of bubonic plague patients who fail to receive any antibiotic treatment die. Untreated septicemic or pneumonic plague is almost always fatal.

Plague Treatment:
A patient diagnosed with suspected plague should be hospitalized and medically isolated. Laboratory tests should be done, including blood cultures for plague bacteria and microscopic examination of lymph gland, blood, and sputum samples. Antibiotic treatment should begin as soon as possible.

The U.S. Public Health Service requires that all cases of suspected plague be reported immediately to local and state health departments and that the diagnosis be confirmed by CDC.

As required by the International Health Regulations, CDC reports all U.S. plague cases to the World Health Organization.