Legionellosis is an infection that is caused by a Legionella species called Legionella pneumophila. The disease has two distinct forms:
Legionnaires' disease is the more severe form of the infection, which may involve pneumonia. The onset of this form of the disease is usually two to 10 days after infection but can occur up to 16 days later. Legionnaires' disease acquired its name in 1976 after an outbreak of pneumonia occurred among people attending the American Legion convention at the Bellevue-Stratford Hotel in Philadelphia. Later, the bacterium causing the illness was named Legionella pneumophila.
Pontiac fever is a milder illness that develops from hours to two days after initial infection and resolves spontaneously. Sometimes, it may not even cause symptoms.
X-ray image of lungs of patient with Legionnaires' disease