Legionellosis is caused by bacteria called Legionella. The disease has two different forms. Legionnaires’ disease is the more severe form of infection that causes pneumonia. Pontiac fever is caused by the same bacteria, but is a milder, self-limiting, acute illness without pneumonia.

The disease got its name after a group of people attending a convention of the American Legion in Philadelphia in 1976 developed pneumonia. While the Legionella bacteria were around before 1976, scientists had never been able to find it in an ill person’s lungs before that time.