Legionellosis describes disease caused by Legionella Sp. Over 50 species of Legionella have been described, although not all are known or suspected human pathogens. Legionella Pneumophilia was the first described and is responsible for most cases of Legionnaires’ disease.
Legionella Sp. are found in water throughout the world, and infection is due to deep inhalation/aspiration of water droplets containing a pathogenic species.
Up to 5% of the general population who inhale Legionella Pneumophilia will develop Legionnaires disease (pneumonia). Up to 95% will develop the less severe influenza-like illness, Pontiac fever. People at increased risk of community acquired Legionnaires disease are older men, smokers and the immunosuppressed.
There has only been one reported case of person-person transmission.