Legionellosis is an infection caused by the bacterium Legionella pneumophila. The disease has two distinct forms:

• Legionnaires' disease, the more severe form of infection which includes pneumonia, and

• Pontiac fever, a milder illness.

People of any age may get Legionnaires' disease, but the illness most often affects middle-aged and older persons, particularly those who smoke cigarettes or have chronic lung disease. Also at increased risk are persons whose immune system is suppressed by diseases such as cancer, kidney failure requiring dialysis, diabetes, or AIDS. Those that take drugs that suppress the immune system are also at higher risk.

Pontiac fever most commonly occurs in persons who are otherwise healthy.

Erythromycin is the antibiotic currently recommended for treating persons with Legionnaires' disease. In severe cases, a second drug, rifampin, may be used in addition. Other drugs are available for patients unable to tolerate erythromycin.

Pontiac fever requires no specific treatment.

Outbreaks of legionellosis have occurred after persons have breathed mists that come from a water source (e.g., air conditioning cooling towers, whirlpool spas, showers) contaminated with Legionella bacteria. Persons may be exposed to these mists in homes, workplaces, hospitals, or public places. Legionellosis is not passed from person to person, and there is no evidence of persons becoming infected from auto air conditioners or household window air-conditioning units.

The Institut National de Santé Publique de Québec published a paper in July, 2003 that showed that 30% of electric water heaters in Quebec are contaminated by Legionella pneumophila. Increasing the water temperature above 60º may kill the Legionella but for safety, to prevent scalding, the temperature should be less than 49º. The Institute was unable to determine how many cases of legionellosis are caused by this contamination.