Infections caused by the bacterium Legionella pneumophila and related Legionella species first emerged in the 1970s associated with contaminated water systems operating at above ambient temperature. The diseases caused by these bacteria are known collectively as legionellosis and range in severity from mild febrile illness (Pontiac fever) to a potentially fatal form of pneumonia (Legionnaires’ disease). Bacteria of the genus Legionellae are ubiquitous in small numbers in many natural water systems, but are particularly well suited to multiplication in warmer artificial aquatic environments in the built environment, such as cooling towers; water systems in hotels, homes, ships and factories; respiratory therapy equipment; fountains; misting devices; and spa baths (also known as spa pools, hot tubs and whirlpool baths).