B. Legionnaire's Disease Legionnaire's Disease, a bacterial pneumonia, is caused by an infection of the lung with Legionella pneumophila. Legionella is ubiquitous in soil and water and may grow in cooling towers of central air-conditioning systems. Infection usually occurs by inhalation of aerosolized bacteria from air-conditioning systems. The disease typically occurs in individuals with a compromised immune system (smoking, alcohol, old age) while healthy children and adults are relatively resistant.