Causes, How it Spreads, and People at Increased Risk
Español: Causas y transmisión
Legionella is a type of bacterium found naturally in freshwater environments, like lakes and streams. It can become a health concern when it grows and spreads in human-made water systems like
- Showers and faucets
- Cooling towers (air-conditioning units for large buildings)
- Hot tubs that aren’t drained after each use
- Decorative fountains and water features
- Hot water tanks and heaters
- Large plumbing systems
Home and car air-conditioning units do not use water to cool the air. They are not a risk for Legionella growth
This bacterium grows best in warm water.
After Legionella grows and multiplies in a building water system, that contaminated water then has to spread in droplets small enough for people to breathe in. People can get Legionnaires’ disease when they breathe in small droplets of water in the air that contain the bacteria.
Less commonly, people can get Legionnaires’ disease by aspiration of drinking water. This happens when water “goes down the wrong pipe,” into the trachea (windpipe) and lungs instead of the digestive tract. People at increased risk of aspiration include those with swallowing difficulties.
In general, people do not spread Legionnaires’ disease and Pontiac fever to other people. However, this may be possible in rare cases. 
Talk to your doctor or local health department if
- You believe you were exposed to Legionella
- You develop symptoms, such as fever, cough, chills, or muscle aches
Your local health department can determine whether or not to investigate. Be sure to mention if you spent any nights away from home in the last two weeks.
Most healthy people exposed to Legionella do not get sick. People at increased risk of getting sick are:
- People 50 years or older
- Current or former smokers
- People with a chronic lung disease (like chronic obstructive pulmonary disease or emphysema)
- People with weak immune systems from diseases like cancer, diabetes, or kidney failure
- People who take drugs that suppress (weaken) the immune system (like after a transplant operation or chemotherapy)
- Correia AM, GonCalves J, Gomes, JP, et al. Probable Person-to-Person Transmission of Legionnaires' Disease. N Engl J Med. 2016;374:497–8.
- Page last reviewed: January 11, 2017
- Page last updated: January 11, 2017
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