Legionella bacteria (Legionella spp.) are water bacteria of which there are more than 40 known species. Legionella pneumophila is the most frequent pathogen of Legionnaire’s disease (legionellosis) which was named after an outbreak among American legionnaires in Philadelphia, USA in 1976. It can be life-threatening as a serious form of pneumonia. Another form of infectious disease caused by legionella bacteria is a disease which is comparable to influenza, so-called Pontiac fever. This usually progresses as a milder disease.