Legionnaires' disease Dr Rob Hicks Contaminated water, especially from heating or water supply systems, transmits the disease.

In this article

What is Legionnaires’ disease? Symptoms

Causes and risk factors Treatment and recovery

Legionnaires' is a bacterial disease that causes a lung infection or pneumonia. It's not contagious (i.e. it cannot be passed from person to person).

Symptoms first appear between 2 and 10 days after exposure to the bacteria. Legionnaires' initially produces a flu-like illness with tiredness, high fever (often 39.5°C or above), headache, muscle aches and a dry cough. As the pneumonia develops there may be chest pain, shortness of breath, abdominal pain, vomiting and diarrhoea, and hallucinations.

There are more than 500 cases a year in the UK – figures for 2006 show 551 reported cases of Legionnaires’ disease in England and Wales, 160 of which were picked up while the affected person was travelling abroad. In about 1 in 10 cases the infection proves fatal.

Legionnaires' is caused by a bacteria known as Legionella pneumophila. The disease and the bacterium were discovered following an outbreak at an American Legion convention in Philadelphia in 1976, hence the name.

The bacteria is found widely throughout natural water systems such as rivers and ponds but temperature is critical to its growth and it is in the warm or hot water of artificial water systems such as heating plants or whirlpools that it can really thrive, forming a biofilm or layer of living bacteria over artificial structures.

Other sources include the water systems of large buildings, cooling towers of air conditioning systems, fountains and ponds, and communal showers.

It used to be thought that people caught Legionella when they breathed in an aerosol or fine mist of contaminated water. But while this may be true in some cases it is now thought that Legionella more commonly occurs when contaminated water in the mouth (drinking water for example) is able to get past the body’s normal defences and passes down into the lungs.

This is known as aspiration and it explains why smokers and those with chronic lung disease are especially vulnerable to Legionella. Normally fluid in the mouth is pushed down the gullet and into the stomach where any particles such as bacteria can be broken down.

The gag reflex prevents water entering into the breathing tubes, and the action of tiny hair-like projections or cilia on the mucosal membranes surface brushes back any particles that have passed towards the lungs.

But in smokers and those with lung disease or weakened immunity these mechanisms may not work properly and bacteria can pass more easily into the lungs to establish a pneumonia.

Occurrences are more common in late summer and early autumn. Men are affected more than women, particularly middle-aged men.

Prompt treatment with antibiotics is effective but specific types of antibiotic must be used as the bacteria can hide inside the cells of the respiratory tract, and the antibiotics must be able to penetrate the cells.

The risk of catching Legionnaires' can be reduced with appropriate maintenance and cleaning of possible sources, such as air conditioning systems.

This article was last medically reviewed by Dr Trisha Macnair in March 2009.