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Legionnaires' disease


Expert reviewer, Professor Robert Read, Professor of Infectious Diseases
Next review due July 2022

Legionnaires’ disease is a severe form of pneumonia caused by Legionella bacteria. You don’t catch it from other people, but by breathing in tiny droplets of contaminated water.

What is Legionnaires' disease?

Legionnaires’ disease first got its name in 1976, when there was an outbreak at a convention of the American Legion in Philadelphia. You may have heard about occasional outbreaks in hotels, on cruises, or around water cooling towers.

Legionnaires’ disease is quite rare in the UK. About 300 to 500 people in England and Wales get it each year and nearly half of these people caught it abroad. Men are more likely to get it than women. It can affect all age groups but most people who get Legionnaires’ disease are over 50.

Legionella bacteria also cause a milder illness called Pontiac fever. For more information about this form of Legionella infection, see our FAQ: What is Pontiac fever? below.

Symptoms of Legionnaires' disease

Legionnaires’ disease is a form of pneumonia. Pneumonia is an infection that causes the small air sacs in your lungs (alveoli) and the tissues around them to become inflamed. The first signs of Legionnaires’ disease usually appear two to 10 days after you’ve been infected. Early Legionnaires’ disease symptoms are similar to those of flu and may include:

  • muscle pains and aches
  • a headache
  • fever (high temperature) and chills
  • a cough that’s dry at first before you cough up phlegm

If your infection gets more severe, you may start with a dry cough and become short of breath. You may have chest pain when you breathe. Some people with Legionnaires’ disease have symptoms such as nausea, vomiting, diarrhoea and abdominal (tummy) pain. Your headache may get worse and you may become confused and unable to think clearly.

These symptoms aren’t always caused by Legionnaires’ disease but if you have them, contact your GP as soon as possible.

Diagnosis of Legionnaires' disease

Your GP will ask you about your symptoms and examine you. They may also ask you about your medical history. Because this infection is quite rare, it’s important that you tell your GP if you’ve been abroad recently or used a spa or hot tub.

Legionnaires’ disease is usually diagnosed with a urine test. Your urine will be tested to check for Legionella antigens. (These are substances that cause your immune system to respond to the infection and produce antibodies). Your doctor may ask you to cough up some mucus, which they’ll send for testing. You may also have a blood test to measure the level of antibodies in your blood.

You might have an X-ray of your chest to check if you have an infection in your lungs. You may have to go to hospital to have this done.

Treatment of Legionnaires' disease

Legionnaires’ disease won’t go away by itself, so it’s important to see your GP for treatment as soon as possible. Legionnaires’ disease is treated with antibiotics.

The sooner you start your treatment, the less likely you are to develop any serious complications. You may need to go into hospital for treatment, particularly if your infection is severe or you’re more at risk of complications. In hospital, you may receive your antibiotic treatment through a drip in your arm before you go on to take tablets.

You’ll usually start to feel better within three to five days. Most people with Legionnaires’ disease make a full recovery, but how long it takes can vary from person to person. This may depend on how severe your condition is, how quickly you start treatment and whether you develop any complications. It may be six months before you feel back to normal.

Causes of Legionnaires' disease

Legionnaires’ disease is caused by Legionella bacteria infecting your lungs. The bacteria can be found naturally in freshwater lakes and streams. They usually only cause a health problem when they get into man-made water supplies where they grow and spread. They grow best in warm temperatures and you usually get infected by breathing in tiny droplets of contaminated water. Potential sources of Legionella bacteria include:

  • showers and taps
  • household plumbing
  • cooling systems (air conditioning units) including cooling towers
  • cold water systems
  • spa baths and hot tubs
  • thermal spas and pools
  • ornamental fountains (that store or recirculate water)
  • hot water systems
  • humidifiers
  • potting compost

Outbreaks of Legionnaires’ disease can take place when these water sources become contaminated with Legionella bacteria. About half of all cases of Legionnaires’ disease are associated with travelling abroad to warmer countries and exposure in an aeroplane or hotel. You can also catch Legionnaires’ disease if you’re in hospital and there’s an outbreak.

Anyone can catch Legionnaires’ disease but you’re more likely to get it if:


You may also be more likely to get Legionnaires’ disease if you have a weakened immune system. For example, if you have HIV/AIDS or are taking medicines that suppress your immune system, such as corticosteroids.

Complications of Legionnaires' disease

If Legionnaires’ disease is left untreated, it can stop your lungs from working properly. If this happens, there won’t be enough oxygen passing from your lungs into your blood. A nurse will give you some extra oxygen, usually through a mask.

Another complication of Legionnaires’ disease is sepsis. This is a severe reaction that can happen if you have a bacterial infection which affects your whole body. You might develop septic shock — this means your blood pressure will drop so your organs won’t receive enough blood to work properly. You’ll need immediate hospital care.

Most people with Legionnaires’ disease are treated successfully but unfortunately, some people do die from the illness.

Frequently asked questions

  • No, there’s no vaccine against the bacteria that cause Legionnaires’ disease. The key to preventing Legionnaires’ disease is making sure that water systems, especially in large buildings, are well maintained by those who own or control them. In the UK and many other countries, there are regulations covering such maintenance.

    Seek medical attention immediately if you think you might have Legionnaires’ disease. Tell your doctor about any foreign travel or potential exposure. Although there’s no vaccine, there are some steps you can take to reduce your risk of catching Legionnaires’ disease.

    • If you smoke, try to stop — you’re more likely to get Legionnaires’ disease if you’re a smoker.
    • If you have a home humidifier or spa/hot tub, follow the manufacturer’s instructions about maintenance and cleaning carefully.
    • If you have a weakened immune system, consider avoiding hot tubs or spas if you can’t tell how well they’re maintained. This may include those in hotels abroad or on cruise ships. For more information about who’s most at risk, see our section: Causes of Legionnaires’ disease above.

  • If you’re an employer, you need to follow health and safety guidelines to reduce the risk of Legionnaires’ disease for your employees. These guidelines are set by the Health and Safety Executive (HSE) and include the following.

    • Identify and assess any sources of risk. This includes checking if present conditions could help bacteria to grow — for example, checking that the water temperature is correct.
    • Prepare a plan to prevent or control an outbreak of Legionnaires’ disease.
    • Systems should be regularly monitored, and a person should be put in charge of the plan.
    • Keep records of what has been done to reduce the risk.
    • If appropriate, tell your local authority that you have a cooling tower in the workplace.

    These guidelines also apply to people in control of buildings, including landlords.

    Visit the Health and Safety Executive website for much more information about your duties and advice on how to carry them out.

  • Yes, you can catch Legionnaires’ disease from a spa bath if it’s not properly maintained or cleaned regularly.

    Legionella bacteria are a particular problem in spa baths because the water is kept at a temperature that they can grow in and multiply. The water is usually vigorously stirred up which sends tiny droplets into the air. These droplets may contain the bacteria which you then breathe in.

    Spa baths should be kept clean and monitored regularly to reduce the risk of an outbreak of Legionnaires’ disease. If you have a spa bath at home, it’s important to follow the manufacturer’s instructions about cleaning and maintenance.

    If you’re unsure whether a spa bath or hot tub has been properly maintained, for example you’re on holiday, consider not using it. You may decide it’s not worth taking the risk, particularly if you’re at higher risk of getting Legionnaires’ disease. For more information, see our section: Causes of Legionnaires’ disease above.

    You can find more information about spa baths and reducing the risks of Legionella infection from the Health and Safety Executive website.

  • Pontiac fever and Legionnaires’ disease are both caused by Legionella bacteria. Pontiac fever got its name from an outbreak in Pontiac, Michigan in 1968.

    Pontiac fever is a mild flu-like illness. Symptoms can include headaches, fever and muscle aches, but unlike Legionnaires’ disease, you won’t have pneumonia.

    Pontiac fever will usually clear up on its own without any treatment within about five days. Because Pontiac fever has similar symptoms to many other viral illnesses, you can probably have it without knowing that’s what you’ve got.

    Doctors don’t know why some people who inhale Legionella bacteria get Pontiac fever and others get Legionnaires’ disease.

    If you have flu-like symptoms that are getting worse after a few days, or you’re concerned about them, contact your GP.


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Related information


    • Legionella infection. BMJ Best Practice. bestpractice.bmj.com, last reviewed July 2019
    • Guidance on investigating cases, clusters and outbreaks of legionnaires’ disease for Public Health England Health Protection Teams. Public Health England. gov.uk, published January 2019
    • Legionnaires' disease. PatientPlus. patient.info/patientplus, last checked 1 February 2017
    • Legionnaires disease. Medscape. emedicine.medscape.com, updated 24 August 2018
    • Legionella and the prevention of legionellosis. World Health Organization. www.who.int, published 2007
    • Sepsis. PatientPlus. patient.info/patientplus, last checked 15 April 2016
    • Legionella (Legionnaires' disease and Pontiac fever). Centers for Disease Control and Prevention. www.cdc.gov, last reviewed 30 April 2018
    • Legionnaires’ disease. The control of Legionella bacteria in water systems. Health and Safety Executive. www.hse.gov.uk, published 2013
    • The control of Legionella and other infectious agents in spa-pool systems. Health and Safety Executive. www.hse.gov.uk, published 2017

  • Reviewed by Rachael Mayfield-Blake, Freelance Health Editor, July 2019
    Expert reviewer, Professor Robert Read, Professor of Infectious Diseases
    Next review due July 2022



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