Legionnaires' disease is a severe form of pneumonia lung inflammation usually caused by infection. Legionnaires' disease is caused by a bacterium known as legionella.
Legionnaires' disease is a type of pneumonia caused by bacteria. You usually get it by breathing in mist from water that contains the bacteria. The mist may come from hot tubs, showers, or air-conditioning units for large buildings. The bacteria don't spread from person to person.