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Background information

The organism

Legionella pneumophila is a relatively common environmental bacterium which lives in free water such as lakes and ponds. It has very simple nutritional requirements and can multiply in pipes and tanks on the scale or sediment left in poorly maintained systems. Legionellae require a temperature range of 20° to 44° Centigrade to multiply, so the bacteria can survive in water systems where the cold water becomes warm, or the hot water is not hot enough (55° Centigrade) to kill the bacteria.

As well as multiplying, the organism needs to be aerosolised to infect humans, this can happen in wet air conditioning systems, in showers and some industrial processes and even when someone turns on a tap and the water splashes against the basin.

Host factors also need to be right for Legionnaires' disease to develop. The people most at risk are men, over the age of 50, with respiratory disease, immunosuppression and who smoke. Legionnaires' disease is almost unknown in children and is much less common in younger people, unless they have the risk factors listed above. The attack rate of Legionnaires' disease is low, less than 10%, so a lot of people need to be exposed to yield a number of clinical cases.

There is also a less severe form of Legionella infection, known as Pontiac fever. Pontiac fever is a mild flu-like illness caused by legionella, often affecting previously healthy and young individuals. Symptoms can include fever, headaches and muscle aches but, unlike Legionnaires' disease, Pontiac fever does not cause pneumonia. The illness will usually resolve without treatment within two to three days. Outbreaks of the disease in the UK are uncommon but have been connected to the inhalation of legionellae found in spa pools. Outbreaks of Pontiac fever have been reported to be caused by L. Pneumophila, L. feeleii, L. micdadei and L. anisa. Pontiac fever may separately, or together with Legionnaires' disease, be referred to as "legionellosis". In contrast to Legionnaires' disease, a high proportion of those exposed to the source of infection may become ill.

Legionnaires' disease

  • Legionnaires' disease was first recognised in 1976 at a convention of the American Legion (Legionnaires) held in Philadelphia
  • The incubation period is 2-19 days (median 6-7 days)
  • It causes severe pneumonia with 10-15% fatality rate
  • It is diagnosed by sputum culture, urinary antigen detection and serology


  • There are roughly 400-500 cases per year in England and Wales, about one third of these are acquired abroad through travel
  • It mainly affects people over 50 years of age
  • Men are three times as likely to get Legionnaires' disease as women
  • People with chronic heart, lung and liver disease are more susceptible, as are people immunosuppressed by disease or treatment
  • Smokers are more susceptible


  • Travel, inside the UK or abroad, including hotels
  • Community, including workplace
  • Hospital exposures
  • Home environment (showers, fountains, spa pools)
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