Legionellosis is a potentially fatal infectious disease caused by Gram negative, aerobic bacteria belonging to the genus Legionella. Over 90% of legionellosis cases are caused by Legionella pneumophila, a ubiquitous aquatic organism that thrives in temperatures between 25 and 45 °C (77 and 113 °F), with an optimum around 35 °C (95 °F).
Legionellosis takes two distinct forms:
- Legionnaires' disease, also known as "Legion Fever", is the more severe form of the infection and produces high fever and pneumonia.
- Pontiac fever is caused by the same bacteria but produces a milder respiratory illness without pneumonia that resembles acute influenza. Pontiac fever also has a spontaneous resolution.