Legionella genus consists of over 50 species of Gram-negative bacteria. The main reservoir of Legionella bacteria are hot water supply systems, refrigeration plants, air conditioners. Legionella bacteria growth  is stimulated by elevated temperatures (about 40ºC), closed water circulation, presence of sediment and biofilm. Strong bacterial growth can occur in enclosed hot water tanks (e.g. boilers), blind tube sections, sediment separators and desulphurisers. Legionella bacteria are a cause of legionellosis in two forms: Legionnaires’ Disease (severe pulmonary disease) and Pontiac fever. No fatalities are observed in the case of Pontiac fever. The main cause of the disease is L. pneumophila (responsible for 80-90% of cases). Other species commonly found in patients are L. micdadei, L. longbeache, L. dumofii and L. bozeman.