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Specialists in the Management of Legionella and Water Quality

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Legionnaires' Disease

What is Legionnaires' Disease?

Legionnaires' disease is a type of pneumonia. It was named after an outbreak of severe pneumonia which affected a meeting of the American Legion in 1976. It is an uncommon but serious disease.

Infographic - male/female ratio

The illness occurs more frequently in men than women and it is thought that this is because of the greater lung capacity in men. It usually affects middle-aged or elderly people and it more commonly affects smokers or people with other chest problems. Legionnaires' disease is uncommon in younger people and is very uncommon in healthy individuals under the age of 20.

About half of all cases of Legionnaires' disease reported in the UK are caught abroad; the other half are the result of infections acquired here.

How is it contracted?

The germ which causes Legionnaires' disease is a bacterium called Legionella pneumophila.

Lungs infographic

Legionnaires' disease is contracted through the inhalation of small droplets of water suspended in the air or the aspiration of water which contains the Legionella bacterium. It can also be contracted by inhaling the aerosols of moisture released when opening vacuum-packed compost bags, as shown by the recent outbreaks in Scotland.

Most people who are exposed to Legionella do not become ill; Legionnaires' disease does not spread from person to person.

Where Does it Come From?

The bacterium which causes Legionnaires' disease is naturally found widespread in nature. It mainly lives in water (rivers, ponds and lakes) but can also be found in wet soil.

Outbreaks occur from purpose-built water systems where temperatures are warm enough (between 25 and 45 degrees Celsius) to encourage growth of the bacteria, for example in water systems, cooling towers, evaporative condensers, whirlpool spas and from water used for domestic purposes in buildings such as hotels.

Community outbreaks in the UK have been linked to installations such as cooling towers or evaporative condensers which can spread droplets of water over a wide area; however the source of the majority of cases (55%) are thought to be associated with domestic water systems. More recently, Legionella bacteria has been found in vacuum-packed compost where it has been responsible for a number of outbreaks, particularly in Scotland

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What are the Symptoms?


  • high temperature, feverishness and chills
  • cough
  • muscle pains
  • headache; and leading on to
  • pneumonia, very occasionally
  • diarrhoea and signs of mental confusion

How is it Treated?

Legionnaires' disease can usually be treated successfully with a course of antibiotics.


Antibiotics that may be used to kill the bacteria include:

  • clarithromycin
  • erythromycin
  • doxycycline
  • azithromycin

What to do?

Health and Safety executive logo

If you develop the symptoms and you are worried that it might be Legionnaires' disease, see your general practitioner.

Because it is similar to the flu, it is not always easy to diagnose. A blood or urine test will be helpful in deciding whether an illness is or is not Legionnaires' disease. When doctors are aware that the illness is present in the local community, they have a much better chance of diagnosing it earlier.

If you suspect that you may have contracted the illness as a consequence of your work, you are advised to report this to your manager, as well as your health and safety representative and occupational health department, if you have one. There is a legal requirement for employers to report cases of Legionnaires' disease that may be acquired at their premises to the Health and Safety Executive.

What Measures are There to Control Legionnaires' Disease?

What measures infographic

To prevent the occurrence of Legionnaires' disease, companies which operate these systems must comply with regulations requiring them to manage, maintain and treat them properly.

The main guideline requirements are to:

  • Identify and assess sources of risk for all water systems regardless of size or volume
  • Prepare a written scheme (or course of action) for preventing or controlling the Risk
  • Implement and manage the scheme - appointing persons to be managerially responsible - the "Responsible Persons" for each pertinent department
  • Keep records and check that what has been done is effective

Further Reading?

To find out more about Legionella and Legionnaires' disease please visit the following sites:

Quick Facts


Legionella bacteria can cause Legionnaires' disease or Pontiac fever, collectively known as Legionellosis.


Although there are at least 56 species of Legionellae, L. Pneumophila is by far the most important human pathogen.






Legionella bacteria are not transmitted from person to person.


Legionnaires’ disease can be contracted when a mist or vapor containing Legionella bacteria is inhaled.


Keeping levels of Legionella bacteria low in water is the key to preventing infection.




The bacterium which causes Legionnaires' disease is widespread in nature. It mainly lives in water, for example ponds, where it does not usually cause problems.


The symptoms of Legionnaires' disease are similar to the symptoms of the flu.