Bug thrives in warm stagnant water

Legionnaire's disease is a bacterial disease which may cause pneumonia. It gets its name from an outbreak at a state convention of the American Legion in Philadelphia, Pennsylvania, in 1976, when 34 of 221 people affected died. But cases have been confirmed as far back as 1947 and it probably occurred earlier.

About 200 cases a year are reported in England, mostly as single isolated incidents, but the bug responsible has been found in ponds, hot and cold water systems, whirlpool baths and water in air conditioning plants. It particularly likes warm stagnant water.

Outbreaks tend to occur in healthy people staying in hotels or other buildings in which cooling systems or showers have been contaminated. The bug may also cause 2% of hospital cases of pneumonia.

Breathing in droplets of water is the most likely route of infection and you cannot catch it from another person. There is also little evidence anywhere in the world that it can be caught from household or car air conditioning systems.

People of all ages can be affected but men over 50 are particularly vulnerable. Early symptoms include flu-like illness, muscle aches, fatigue, headaches, dry cough and fever. Sometimes diarrhoea and confusion can develop, and the symptoms frequently lead to pneumonia. The illness is treated by antibiotics but deaths occur in 10%-15% of normally healthy individuals. The rate can be higher in some groups of patients, including older people, heavy smokers and drinkers and those who are already ill, with diseases such as cancer, kidney failure, diabetes or Aids. Irreparable damage to the lungs is the most usual cause of death.

Symptoms of Legionnaire's, officially caused by a bug called legionella pneumophila, can take up to 10 days to develop, or, in very rare cases, three weeks. Three to six days is more normal.

Rapid diagnosis is possible through testing patients' urine samples.