What is Legionnaires’ disease?

Legionnaires’ disease, or legionellosis, is a common type of bacterial pneumonia (lung infection) that can be very serious and sometimes life-threatening. A particular strain of bacteria called Legionella causes the illness.

Who is most at risk for Legionnaires’ disease?

Older people and people with compromised immune systems are most at risk of getting Legionnaires’ disease. The healthier you are, the less chance you’ll get Legionnaires’ disease (even if you breathe in the bacteria).

You have an increased risk of getting Legionnaires’ disease if you:

How does someone get Legionnaires’ disease?

Legionella bacteria live best in warm water. They are found in some freshwater lakes and streams. When certain man-made water structures aren’t maintained with proper disinfectants, the bacteria can grow and cause a health risk. But it has also been found in natural water and even soil. The man-made water systems most susceptible to Legionella spread include:

  • Hot tubs
  • Shower heads
  • Faucets
  • Humidifiers
  • Decorative water features, such as fountains
  • Hot water tanks and heaters
  • Air conditioning units for large buildings

Legionella bacteria become airborne in tiny droplets of water rising from these water systems. When someone breathes in air that contains the bacteria, they can get Legionnaires’ disease.

Less often, people can contract Legionnaires’ disease by aspirating (breathing in) drinking water. This is when water “goes down the wrong pipe,” sending liquid down the windpipe and into your lungs instead of down your esophagus and into your stomach. Legionnaires’ disease cannot be spread from person to person.

What are the most common symptoms of Legionnaires’ disease?

Legionnaires’ disease symptoms look a lot like other pneumonia symptoms. If you have Legionnaires’ disease, you may experience:

Most people see symptoms start a few days up to a week after being exposed to the Legionella bacteria. Sometimes, symptoms may show up 2 weeks after exposure.

Last reviewed by a Cleveland Clinic medical professional on 03/19/2018.


  • Centers for Disease Control and Prevention. Legionella. Accessed 4/9/2018.
  • American Thoracic Society. What is Legionnaires’ Disease. Accessed 4/9/2018.
  • Dooling KL, Toews KA, Hicks LA, et al. Active Bacterial Core Surveillance for Legionellosis–United States, 2011–2013. MMWR Morb Mortal Wkly Rep. 2015;64(42):1190–3

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