About Legionellosis

Legionellosis is a disease caused by Legionella bacteria, and can take one of two forms: Legionnaire’s disease and Pontiac fever.  People with either of these illnesses may have appetite loss, malaise, muscle pain, headache, and fever.  However, Legionnaires’ disease is associated with pneumonia while Pontiac fever is a milder disease that does not cause pneumonia.

Legionnaires' disease is caused by a type of bacteria called Legionella. The bacteria received its name in 1976, when many people who went to a Philadelphia convention of the American Legion suffered from an outbreak of this disease, a type of pneumonia. Each year, between 8,000 and 18,000 people are hospitalized with Legionnaires' disease in the U.S. However, many infections are not diagnosed or reported, so this number may be higher. More illness is usually found in the summer and early fall, but it can happen any time of year.

Causes and Transmission

The Legionella bacteria are found naturally in the environment, usually in water. The bacteria grow best in warm water, such as water in hot tubs, cooling towers, hot water tanks, large plumbing systems, or parts of the air-conditioning systems of large buildings.

People get Legionnaires' disease when they breathe in a mist or vapor (small droplets of water in the air) that has been contaminated with the bacteria. One example might be from breathing in the steam from a whirlpool spa that has not been properly cleaned and disinfected.  The bacteria are NOT spread from one person to another person.

Outbreaks of Legionnaires' Disease

Outbreaks are when two or more people become ill in the same place at about the same time, such as patients in hospitals. Hospital buildings have complex water systems, and many people in hospitals already have illnesses that increase their risk for Legionella infection.

Other outbreaks have been linked to aerosol sources in the community, or with cruise ships and hotels, with the most likely sources being whirlpool spas, cooling towers (air-conditioning units from large buildings), and water used for drinking and bathing.

People at Risk

People most at risk of getting sick from the bacteria are older people (usually 65 years of age or older), as well as people who are smokers, or those who have a chronic lung disease (like emphysema).

People who have weak immune systems from diseases like cancer, diabetes, or kidney failure are also more likely to get sick from Legionella bacteria. People who take drugs to suppress (weaken) the immune system (like after a transplant operation or chemotherapy) are also at higher risk.


Prevention includes stopping smoking if you smoke to decrease your risk of disease, disinfecting cooling towers according to recommended guidelines and cleaning whirlpools and hot tubs according to manufacturers’ recommendations.

Legionellosis Resource Site (CDC)