Legionnaires' disease is a potential fatal pneumonia caused by legionella bacteria. Infection is caused by breathing in small droplets of water contaminated by the bacteria. The disease cannot be passed from one person to another.

Everyone is potentially susceptible to infection but some people are at higher risk' e.g. those over 45 years of age, smokers and heavy drinkers, those suffering from chronic respiratory or kidney disease and people whose immune system is impaired.

The bacteria survives at low temperatures and thrive at temperatures between 20-45°C if the conditions are right, e.g. if a supply of nutrients is present such as rust, sludge, scale, algae and other bacteria. They are killed by high temperatures or by chemical treatment.

Legionella bacteria are common in natural water courses such as rivers and ponds. Since legionella are widespread in the environment, they may grow in other water systems such as cooling towers and hot and cold water services.