- Legionella can cause Legionnaires’ disease and Pontiac fever, collectively known as legionellosis.
- Scientists named the bacteria 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. A recent study estimated that the true number of Legionnaires’ disease cases may be 1.8–2.7 times higher than what is reported.2
- About one in 10 people who gets sick from Legionnaires’ disease will die.2,3
- 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.3,4
- Legionella occurs naturally in freshwater environments, like lakes and streams. It can become a health concern when it grows and spreads in human-made building water systems.
- Reported cases of notifiable diseases, by region and area – United States and U.S. territories, 2016.
- Collier SA, Deng L, Adam EA, et al. Estimate of burden and direct healthcare cost of infectious waterborne disease in the United States. Emerg Infect Dis. 2021;27(1):140–9.
- 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.
- Correia AM, GonCalves J, Gomes, JP, et al. Probable person-to-person transmission of Legionnaires’ diseaseexternal iconexternal icon. N Engl J Med. 2016;374:497–8.
Page last reviewed: March 25, 2021