Overview

Legionnaires' disease is a severe form of pneumonia — lung inflammation usually caused by infection. It's caused by a bacterium known as legionella.

Most people catch Legionnaires' disease by inhaling the bacteria from water or soil. Older adults, smokers and people with weakened immune systems are particularly susceptible to Legionnaires' disease.

The legionella bacterium also causes Pontiac fever, a milder illness resembling the flu. Pontiac fever usually clears on its own, but untreated Legionnaires' disease can be fatal. Although prompt treatment with antibiotics usually cures Legionnaires' disease, some people continue to have problems after treatment.