Legionella pneumophila, a gram negative non spore forming motile bacillus, is the etiologicagent of Legionellosis.

Clinical symphtoms can range in severity from a mild, febrile illness(Pontiac fever) to a rapid and potentially fatal pneumonia (Legionnaires’ disease). L. pneumophila is a common cause of nosocomial and travel-acquired pneumonia and with thebacterial species Mycoplasma pneumoniae and Clamydia pneumoniae is one of the top three cause of sporadic, community acquired pneumonia. The first strain of legionella were isolated in 1943 and classified as Riccktesia like organism, and the genus was established in 1979 after a large outbreak of pneumonia among members of the American legion that occurred 3 years earlier. After this, a large spectrum of legionella species was classified,actually genus includes 50 species and 16 serogrups of L. pneumophila were classified. Legionellae are ubiquitous in natural and artificial water environments worldwide, and survive in a range of environmental conditions.