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Fast Facts

Español: Datos breves

  • 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.[1] 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. [2]
  • 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. [2]
  • 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.



  1. CDC. Notice to Readers: Final 2015 Reports of Nationally Notifiable Infectious Diseases and Conditions. MMWR Morb Mortal Wkly Rep. 2016;65(46):1306–21.
  2. Correia AM, GonCalves J, Gomes, JP, et al. Probable Person-to-Person Transmission of Legionnaires’ Disease. N Engl J Med. 2016;374:497–8.