Legionnaires’ disease is a type of pneumonia caused by inhaling infected water droplets containing the bacterium L. pneumophila. It was named after an outbreak of severe pneumonia that affected a meeting of the American Legion in 1976. It is an uncommon but serious disease.

It is actually one of a group of similar diseases collectively known as legionellosis. The other forms, namely, Pontiac Fever and Lochgoilhead Fever, have similar symptoms but are not as serious as Legionnaires’ disease.

Legionnaires’ disease occurs more frequently in men than women. It usually affects middle-aged or elderly people, and it more commonly affects smokers or people with other chest problems.

Certain conditions increase the risk from legionella:

  • a suitable temperature for growth, 20 to 45oC;
  • a source of nutrients for the organism, e.g. sludge, scale, rust, algae, and other organic matter; and a way of creating and spreading breathable droplets, e.g. the aerosol created by a cooling tower or spa pool.

Most people exposed to legionella do not become ill, and Legionnaires’ disease does not spread from person to person.

The symptoms of Legionnaires’ disease are similar to those of flu: high temperature, fever and chills; cough; muscle pains; and headache.

In extreme cases there may also be pneumonia, and occasionally diarrhoea and signs of mental confusion.

Legionella bacteria are widespread in nature, mainly living in natural water systems, e.g. rivers and ponds. However, the conditions are rarely ideal for people to catch the disease from these sources.

Outbreaks of the illness occur from exposure to legionella growing in purpose-built systems where the water is maintained at a temperature high enough to encourage growth, e.g. cooling towers, evaporative condensers, spa pools, and hot water systems used in all types of premises (work and domestic).

Most community outbreaks in the UK have been linked to installations such as cooling towers, which can spread droplets of water over a wide area. These are found as part of air-conditioning and industrial cooling systems.