Fast Facts

  • Legionella can cause Legionnaires’ disease, Pontiac fever, and, more rarely, extrapulmonary infections, collectively known as legionellosis.
  • Scientists named the bacterium after an outbreak in Philadelphia in 1976. During that outbreak, many people who went to an American Legion convention got sick with pneumonia (lung infection).
  • Health departments reported nearly 10,000 cases of Legionnaires’ disease in the United States in 2018.1 However, because Legionnaires’ disease is likely underdiagnosed, this number may underestimate the true incidence.
  • About one in 10 people who gets 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 under rare circumstances.3
  • Legionella occurs naturally in fresh water environments, like lakes and streams. It can become a health concern when it grows and spreads in human-made building water systems.
  • Keeping Legionella out of water systems in buildings is key to preventing infection.


  1. Reported cases of notifiable diseases, by region and area – United States and U.S. territories, 2016.
  2. 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.
  3. Correia AM, GonCalves J, Gomes, JP, et al. Probable person-to-person transmission of Legionnaires’ diseaseexternal icon. N Engl J Med. 2016;374:497–8.