Legionella is a water-based organism that causes infection in humans when they inhale it in aerosol form. It can cause a severe, progressive form of pneumonia that is fatal in up to 15% of the cases. The disease was first identified in 1978 after an epidemic outbreak among American Legion conventioneers in Philadelphia in 1976 infected 221 and killed 34. The agent was described as a rod-shaped bacteria which was named legionella pneumophila.
According to Jan De Baat Doelman, president of Scalewatcher North America, Inc., Oxford, PA, besides cooling towers, legionella can be found in a number of systems, including potable hot and cold water systems, recirculating water humidifiers, whirlpool spas, and others.