Legionnaires' disease, which can cause a potentially fatal form of pneumonia, got its name when a serious outbreak struck delegates attending an American Legion conference in Philadelphia in 1976.
Within days, 29 legionnaires had died and 180 had pneumonia. The condition was identified and named Legionella pneumophila.
A group of bacteria commonly found in harmless numbers in water, legionellae can multiply to high levels in stagnant water. When the temperatures reaches 20 to 45 degrees, the bacteria flourishes.
Chlorination of water supplies does not guarantee elimination of legionellae. The bug is found in hot and cold water systems, cooling towers of air conditioning units, spas, jacuzzis and thermal springs.
The common denominator in all sources is the presence of water at high temperatures and the potential to form aerosols (tiny droplets of water). Legionnaires' disease is usually contracted by inhaling these aerosols. Person-to-person transmission has never been proven.
It is particularly serious for older people, those who are immuno-compromised and for heavy smokers. It can have a death rate of 10 per cent.