Definition of Plague:

Plague is a severe and potentially deadly bacterial infection.

Causes, incidence, and risk factors:

Plague is caused by the bacteria Yersinia pestis. Rodents, such as rats, carry the disease. It is spread by their fleas.

People can get the plague when they are bitten by a flea that carries the plague bacteria from an infected rodent. In rare cases, you may get the disease when handling an infected animal.

A plague lung infection called pneumonic plague can spread from human to human. When someone with pneumonic plague coughs, tiny droplets carrying the bacteria move through the air. Anyone who breathes in these particles may catch the disease. An epidemic may be started this way.

In the Middle Ages in Europe, massive plague epidemics killed millions of people. Plague can still be found in Africa, Asia, and South America.

Today, plague is rare in the United States, but it has been known to occur in parts of California, Arizona, Colorado, and New Mexico.

There three most common forms of plague are:

• Bubonic plague -- an infection of the lymph nodes
• Pneumonic plague -- an infection of the lungs
• Septicemic plague -- an infection of the blood

The time between being infected and developing symptoms is typically 2 to 7 days, but may be as short as 1 day for pneumonic plague.

Risk factors for plague include a recent flea bite and exposure to rodents, especially rabbits, squirrels, or prairie dogs, or scratches or bites from infected domestic cats.