Plague is an illness resulting from infection with the bacterium Yersinia pestis. Left untreated, it is potentially life-threatening. Plague cases occur only rarely in the United States and are most likely in northern New Mexico, northern Arizona, southern Colorado, southern Oregon, western Nevada and California.

There are three major types of plague:

• Bubonic plague. Historically the most common type of plague to affect humans, it is usually caused by a bite from an infected flea.
  
  
• Septicemic plague. Results from plague bacteria that multiply in the bloodstream. This may occur directly from a flea bite or may result as a complication of bubonic or pneumonic plague.
  
  
• Pneumonic plague. The least common form of plague, it is nonetheless the most fatal. People typically get pneumonic plague from inhaling infectious droplets coughed into the air by infected people or animals.

Plague cases are most likely to occur in rural areas and urban areas with poor sanitation, large rat populations and overcrowding. Incidences may occur at any time of the year, but are most likely between April and November in the United States.

Symptoms of plague may vary somewhat depending on the nature of the infection. For example, patients with bubonic plague may experience painful, swollen lymph nodes known as buboes. Patients with septicemic plague may experience bleeding from the mouth, nose, rectum or under the skin. Meanwhile, patients with pneumonic plague may have symptoms that resemble those of pneumonia, including bloody sputum, breathing difficulties and chest pain.

Because symptoms of plague can mimic those of other illnesses, various tests may be performed to confirm a plague diagnosis. Once a plague diagnosis has been made, the patient is usually isolated and local and state health departments are notified of the potential health threat to the community. Early treatment is crucial to survive any form of plague. This typically involves intravenous antibiotics.

Controlling rat populations is the best method of preventing plague, because rats and other rodents act as hosts for the bacteria. Antibiotics can be taken as a preventive measure by people bitten by fleas or exposed to the tissues or fluids of animals infected with plague.