Legionella is a pathogenic bacteria that causes legionellosis, a respiratory disease in humans. Sometimes the bacteria cause a serious type of pneumonia (lung infection) called Legionnaires’ disease. The bacteria can also cause a less serious infection called Pontiac fever that has symptoms similar to a mild case of the flu.

Legionella is a type of bacterium found naturally in freshwater environments, like lakes and streams. It can become a health concern when it grows and spreads in human-made water systems such as showers and faucets, cooling towers (air-conditioning units for large buildings), hot tubs that aren’t drained after each use, decorative fountains and water features, hot water tanks and heaters, large plumbing systems. That contaminated water then has to spread in droplets small enough for people to breathe in.