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 2018.1However, 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.