Legionellosis (Legionnaires' Disease)  Information for adults

Legionellosis is the name for infection caused by the Legionella pneumophila bacterium. Infection can take two forms: the more serious legionnaires' disease, which is a pneumonia, and the less serious Pontiac fever. The L. pneumophila bacterium got its name from an outbreak at a convention of the American Legion in 1976, when many people staying in the same hotel developed an unusual pneumonia. L. pneumophila is found naturally in water and is often found in the water systems of large buildings, such as the heating and cooling elements of hotels and hospitals.

Legionellosis is transmitted by inhaling water droplets that are contaminated with L. pneumophila. There is no spread of legionellosis by person-to-person contact. Examples of sources of contaminated water sources include air conditioning units (usually those servicing very large buildings, not small window units) or steam from a sauna or whirlpool.

Because the infection is somewhat difficult to diagnose, and because common treatments for other respiratory infections will successfully treat legionellosis (even without a diagnosis ever being made), many cases of legionellosis probably go unrecognized. Usually, it is not a serious infection, though in patients who are already ill or who are at risk, mortality may be 30%.