Plague is an infectious disease caused by the bacterium Yersinia pestis. The bacterium is found mainly in rodents, particularly rats, and in the fleas that feed on them. Other animals and humans usually contract plague from rodent or flea bites. Historically, plague decimated entire civilizations. In the 1300s, the 'Black Death,' as it was called, killed approximately one-third (20-30 million) of Europe's population. In the mid-1800s, it killed 12 million people in China. Today, thanks to better living conditions, antibiotics, and improved sanitation, there are only about 1,000 to 3,000 cases a year worldwide.
Yersinia pestis is found in animals throughout the world, most commonly in rats but occasionally in other wild animals, such as prairie dogs. Most cases of human plague are caused by bites of infected animals or the infected fleas that feed on them. In almost all cases, only the pneumonic form of plague (see below) can be passed from person to person. Only one plague bacterium causes plague, but it can infect people in three different ways. In bubonic plague, the most common form, plague bacteria infect the lymph system. Septicemic plague - this form of plague occurs when the bacteria multiply in the blood. Pneumonic plague - this is the most serious form of plague and occurs when Y. pestis bacteria infect the lungs and cause pneumonia.
When the disease is suspected and diagnosed early, health care workers can treat people with plague with specific antibiotics, generally streptomycin or gentamycin. Certain other antibiotics are also effective. Left untreated, bubonic plague bacteria can quickly multiply in the bloodstream, causing septicemic plague, or even progress to the lungs, causing pneumonic plague. Currently, there is no commercially available vaccine against plague. Approximately 10 to 20 people in the United States develop plague each year from flea or rodent bites-primarily infected prairie dogs-in rural areas of the southwestern United States. About 1 in 7 of those infected die from the disease. There has not been a person-to-person infection in the United States since 1924.