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Causes, How it Spreads, and People at Increased Risk

Español: Causas y transmisión

Causes and Common Sources of Infection

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

  • Hot tubs that aren’t drained after each use
  • Hot water tanks and heaters
  • Large plumbing systems
  • Cooling towers (air-conditioning units for large buildings)
  • Decorative fountains

Home and car air-conditioning units do not use water to cool the air, so they are not a risk for Legionella growth.

This bacterium grows best in warm water.


How it Spreads

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 are exposed to Legionella when they breathe in mist (small droplets of water in the air) containing the bacteria. One example might be from breathing in droplets sprayed from a hot tub that has not been properly cleaned and disinfected.

Less commonly, Legionella can be spread by aspiration of drinking water, which is when water “goes down the wrong pipe,” into the trachea (windpipe) and lungs instead of down the digestive tract. People at increased risk of aspiration include those with swallowing difficulties. In general, Legionnaires’ disease and Pontiac fever are not spread from one person to another. However, this may be possible in rare cases. [1]

If you have reason to believe you were exposed to the bacteria, talk to your doctor or local health department. Your local health department can determine if an investigation is needed. Be sure to mention if you spent any nights away from home in the last two weeks.


People at Increased Risk

Most healthy people do not get sick after being exposed to Legionella. 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 a weak immune system 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)


  1. Correia AM, GonCalves J, Gomes, JP, et al. Probable Person-to-Person Transmission of Legionnaires' Disease. N Engl J Med. 2016;374:497–8.

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