Legionellosis is the collective name given to the pneumonia-like illness caused by Legionella bacteria. This includes the most serious Legionnaires’ disease caused by Legionella pneumophila, as well as the similar but less serious conditions of Pontiac fever and Lochgoilhead fever. Humans are accidental hosts of the Legionella bacteria and infection occurs as a result of breathing in contaminated water droplets in the form of aerosols. The bacteria enter the lungs where conditions are favourable for growth. Particularly vulnerable are the elderly, the immunosuppressed and smokers.