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- Legionella can cause Legionnaires’ disease or Pontiac fever, collectively known as legionellosis.
- The bacterium was named after an outbreak in 1976, when many people who went to a Philadelphia convention of the American Legion got sick with pneumonia (lung infection).
- About 6,000 cases of Legionnaires’ disease were reported in the United States in 2015. However, because Legionnaires’ disease is likely underdiagnosed, this number may underestimate the true incidence.
- About one out of every 10 people who get sick from Legionnaires’ disease will die. 
- People can get Legionnaires’ disease or Pontiac fever when they breathe in small droplets of water in the air that contain Legionella.
- In general, people do not spread Legionnaires’ disease to other people. However, this may be possible in rare cases. 
- Legionella is found naturally in fresh water environments, like lakes and streams. It can become a health concern when it grows and spreads in human-made water systems.
- Keeping Legionella out of water systems in buildings is key to preventing infection.
- Page last reviewed: June 1, 2017
- Page last updated: June 1, 2017
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