Legionnaires' disease

Page last reviewed: 13/07/2011

Legionnaires' disease is a potentially fatal lung infection (pneumonia) that is caused by the legionella bacteria. Legionnaires' disease is caught by breathing in small droplets of contaminated water. It is not contagious and cannot be spread directly from person to person.

Initial symptoms include a high fever and muscle pain. Once the bacteria begin to infect your lungs, you may also develop a persistent cough (see Legionnaires' disease - symptoms). Prompt treatment using antibiotics is essential to reduce the risk of death. See Legionnaires' disease - treatment for more information.

The condition is called Legionnaires' disease because it was first identified after a large outbreak at a hotel hosting a convention of a veteran organisation known as the American Legion.

Causes of Legionnaires' disease

Legionella bacteria is commonly found (often in low numbers) in sources of water, such as rivers and lakes. The bacteria sometimes find their way into artificial water supply systems such as:

  • air conditioning systems
  • hot and cold water services
  • cooling towers

Given the right conditions, legionella bacteria can rapidly spread and contaminate these water systems.

Large buildings, such as hotels, hospitals, museums and office blocks, are more vulnerable to legionella contamination because they have larger, more complex water supply systems in which legionella contamination can quickly spread.

There are strict guidelines regarding the maintenance and control of water supply systems, such as either keeping the water cooled below 20ºC (68ºF) or heated above 60ºC (140ºF) to prevent an outbreak of Legionnaires' disease.

See Legionnaires' disease - causes for more information about where legionella bacteria can be found and how they can spread.

How common is Legionnaires' disease?

In 2010, there were 11 reported cases of Legionnaires' disease in Ireland, of which 7 were travel acquired.

However, the actual number of cases of Legionnaires' disease may be much higher than the reported number of cases. This is because not everyone with pneumonia may have the relevant test for Legionnaires' disease, particularly those with milder symptoms. Therefore, these cases may not be reported to the health authorities.

Legionnaires' disease is three times more common in men than women, and it mostly affects people who are over 50 years old.


Legionnaire's disease can be very serious, particularly in vulnerable people, such as the elderly or people with pre-existing health conditions.

An estimated 10-15% of otherwise healthy people who contract Legionnaires' disease will die. The number of deaths may be higher in people with pre-existing health conditions, such as a weakened immune system (the body's natural defence against illness and infection).

Page last reviewed: 13/07/2011

The symptoms of Legionnaires' disease can begin any time from two to 19 days after exposure to the initial infection. However, six to seven days is the most common time between getting the infection and the onset of symptoms (incubation period).

Symptoms usually begin with an initial phase that lasts one to two days, in which you experience:

  • mild headaches
  • muscle pain

This is followed by the onset of more severe symptoms, including:

  • high fever, sometimes a temperature of 38ºC (100.4ºF) or above
  • more severe muscle pain
  • chills
  • tiredness 
  • changes to your mental state, such as confusion

Once the bacteria begin to infect your lungs, you may also experience:

  • a persistent cough, which is usually dry at first but as the infection develops you may start coughing up mucus or, rarely, blood
  • shortness of breath
  • chest pains

Some people with Legionnaires' disease also have symptoms that affect the digestive system, including:

  • feeling sick
  • being sick
  • diarrhoea
  • loss of appetite

When to seek medical advice

The symptom of a high fever is almost always caused by an infection. If you have a high fever, contact your GP as soon as possible to consider the various possibilities.

Page last reviewed: 13/07/2011

Legionella bacteria can be found in any freshwater environment, such as rivers and lakes. The bacteria are usually only present in low numbers because the temperature of the water is often too low for the bacteria to grow and spread.

However, if the bacteria manage to find their way into an artificial water system, given the right circumstances, they can quickly grow and reproduce, leading to a widespread contamination of the water system.

The three things that the legionella bacteria require to grow and reproduce are:

  • a water temperature of between 20-45ºC (68-113ºF)
  • impurities in the water that the bacteria can use for food, such as rust, sludge, algae and limescale
  • water stagnation, e.g. a dead leg in a plumbing system or water outlets which are used less than once weekly.

Water systems that are known to be vulnerable to legionella contamination include:

  • hot and cold water systems for large buildings, such as hotels and hospitals
  • air conditioning systems that use water for cooling purposes
  • cooling towers

However, any artificial water system is potentially vulnerable to contamination. For example, cases of Legionnaires' disease have reportedly arisen from contaminated:

  • baths and showers
  • fountains
  • sprinkler systems
  • whirlpool baths
  • spas
  • humidifiers that were being used in food display cabinets

You can catch Legionnaire's disease by inhaling small droplets of contaminated water mist in the air. There is no evidence of spread from person-to-person.

Risk factors

Everyone is potentially at risk of developing Legionnaires' disease. However, a number of risk factors make it more likely for certain individuals to experience a more severe form of the infection. These risk factors include:

  • being 40 years of age or over
  • being male
  • being a smoker or having a past history of heavy smoking
  • being a heavy drinker
  • having diabetes
  • having kidney disease
  • having a weakened immune system (the body's natural defence against infection and illness) - for example, people with HIV and AIDS or those who have had an organ transplant 
  • having cancer, particularly lung cancer or leukaemia (cancer of the white blood cells)
  • having a pre-existing lung condition, such as chronic obstructive pulmonary disease
  • having a heart condition, such as heart failure  
  • having a liver condition, such as alcoholic liver disease  

Page last reviewed: 13/07/2011

See your GP if you are concerned that you have symptoms that you think may be the result of Legionnaires' disease.

You should inform your GP if you have recently spent time in a building that could be vulnerable to a legionella infection, such as a hotel or a hospital. Also let them know if you have travelled away from home and stayed in a hotel in Ireland or abroad. It is estimated that around two thirds of Legionnaires' disease cases are caught outside Ireland.

This information will be helpful in confirming a diagnosis and possibly allowing the health authorities to pinpoint the source of the infection.

Urine test

Legionnaires' disease can be diagnosed using a urine test. The Legionella bacteria shed proteins called antigens, which can be detected in a urine sample.

Further tests

Further tests, such as a blood test, sputum test or a chest X-ray, may be recommended to assess the effect of the infection on your overall health and on organs, such as your lungs and kidneys.

Notifiable disease

Legionnaires' disease is a notifiable disease. This means that if a doctor diagnoses the condition, they must tell the local Director of Public Health under the Public Health (Infectious Diseases) Regulations . The authority will try to identify the source of the outbreak and put in place any necessary precautionary measures.

If you are able to provide the name of the hotel you stayed in while abroad it is important that you do so. Systems of communication are in place to make sure that the details of the hotel you stayed in are passed on to the Public Health authorities in that country. The Public Health authorities in the country of the hotel can them make sure that the hotel is inspected so that they can put in place any necessary precautionary measures.

Page last reviewed: 13/07/2011


Legionnaires' disease is treated with antibiotics to kill the bacteria. Possible antibiotics that may be used include:

  • clarithromycin
  • fluoroquinolene

These may be taken by mouth as tablets or capsules or they may be given through an intravenous infusion. An intravenous infusion is where the medicine is given in hospital by a continuous drip through a narrow tube into a vein in your arm.

Depending on the severity of your condition, you will usually need to take antibiotics for 7 to 10 days, although in some cases you may need to take them for up to three weeks. 

Side effects

Possible side effects of the antibiotics that are used to treat Legionnaires' disease include:

  • dizziness
  • feeling sick
  • being sick
  • diarrhoea
  • loss of appetite
  • headaches
  • drowsiness
  • shortness of breath

Treatment in hospital

If you have pre-existing risk factors that make you more vulnerable to the effects of Legionnaires' disease, such as being elderly or having diabetes, it is likely that you will be admitted to hospital. This is so that the functions of your body can be supported while you recover from the infection.

You may be given oxygen and your breathing will be supported if necessary. You may also be given intravenous fluids (fluids directly into a vein in your arm) to prevent you becoming dehydrated. Your heart rate and blood pressure may also be monitored.

Page last reviewed: 13/07/2011

In particularly severe cases of Legionnaires' disease, a number of life-threatening complications can occur. These include:

  • the lungs are unable to provide the body with enough oxygen (respiratory failure)
  • the kidneys do not work, which can lead to a dangerous build-up of fluids and waste in the blood (kidney failure)
  • a blood infection, leading to a sudden and dangerous drop in blood pressure (septic shock)

An estimated 10-15% of people with Legionnaires' disease will die from the condition.

Page last reviewed: 13/07/2011

The best way to prevent an outbreak of Legionnaires' disease is to ensure that any water system under your control is properly maintained and conforms to the relevant health and safety regulations.

The two most important factors for preventing an outbreak of Legionnaires' disease are:

  • temperature - any water in the system should either be cooled to below 20ºC (68ºF) or heated to above 60ºC (140ºF)
  • hygiene - the water should be kept free of any impurities and never allowed to stagnate (lose its freshness from not moving)

If you are an employer or a private landlord, you have a legal duty to ensure that all water systems in your premises are properly operated and maintained to prevent Legionnaires' disease or any other type of water-borne infection.

If you are a homeowner, you are responsible for the water systems in your house. In theory, these could also cause an outbreak of Legionnaires' disease if they are not properly maintained. However, in practice, the risk is low if you keep your hot water at a high enough temperature and regularly use your water systems (to prevent stagnation).

Travelling abroad

If you are travelling abroad, be aware of the risk of Legionnaires' disease, and seek medical advice if you develop any of the symptoms. 

At-risk groups

If you are at increased risk of developing severe Legionnaires' disease - for example, because you have a weakened immune system, you may want to consider avoiding water systems that could be contaminated, such as spas.

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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