Causes of Legionnaire's disease

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Legionella bacteria are found naturally in the environment and thrive in warm water and warm damp places. They live in fresh water of streams, lakes and thermal springs, moist soil, and mud. It can grow inside layers of slime in pipes, and on plants and rocks. It has great powers of survival. In their normal environment Legionella bacteria probably do not cause infections but man-made water systems sometimes provide environments that let Legionella bacteria increase to large numbers causing outbreaks.

These man-made systems include cooling towers associated with air conditioning and industrial cooling processes, showers or spa pools. Legionella bacteria tend to grow in large, warm, moist areas such as air‑conditioning towers. They are spread into the air in water droplets. The bacteria prefer temperatures of 20 to 45 Â°C. Known sources therefore include:

Re-circulating water in air-conditioning and cooling systems,
Whirlpool spas and other warm-water baths,
Decorative fountains,
Water reservoirs of the humidifiers and nebulisers of respiration machines used in hospital to maintain patientsâ€™ breathing,
and Cooling towers.

Infection occurs when a person breathes in water droplets that are contaminated by many legionella bacteria.

Individual cases also sometimes occur and the source of the bacteria is not found.

Legionnaires' disease is not passed from person to person, nor from drinking water contaminated by Legionella bacteria.