Legionnaires’ disease is a serious life-threatening infection of the lungs, usually caused by the bacterium Legionella pneumophila (in 90% of the cases). It is a lethal form of atypical pneumonia. The bacteria can also cause a milder form of this disease called Pontiac Fever.
This species of bacteria is usually present in freshwater reservoirs like streams and lakes. People can catch it by inhaling in environments where bacteria have got their way into the water supply. Hot tubs, hot water tanks, cooling towers of large air conditioners, large plumbing systems, decorative fountains, showers, faucets, etc. can easily be contaminated by the bacteria.
However, car and home air-conditioners do not use water for cooling the air so they are not usually a source of Legionella’s growth.
It can also infect a person if the contaminated water is mistakenly aspirated by him.