What Is Legionnaires' Disease

  • 1st Revision: Bea Brownlee


Legionellosis is the collective name of two respiratory infections caused by the Legionella bacteria. The first and most common of these two conditions is called Legionnaires’ disease, a severe pneumonia requiring hospital treatment. The second is Pontiac fever, a milder, influenza-like illness that does not cause pneumonia.1

Legionella bacteria were discovered after an outbreak in 1976. Attendees at a convention in Philadelphia became ill with a type of pneumonia that is now known as Legionnaires’ disease.2 Legionnaires’ disease is fatal in around 10% of cases. 

Causes of legionnaires' disease

There are at least 60 types of Legionella bacteria, most of which can cause disease. These bacteria are gram-negative. Gram-negative bacteria are often the cause of infections like pneumonia, blood infections and meningitis in healthcare settings, but are difficult to treat because they are becoming increasingly resistant to antibiotics.3

The particular species that causes most cases of legionellosis  is called Legionella pneumophila. Along with other types of Legionella bacteria, it is commonly found in small quantities in natural water sources such as rivers and lakes. It’s unlikely for humans to contract legionellosis from these sources, because only a small amount of bacteria is present, and conditions are not optimal for growth of the bacteria. 

Man-made water systems offer more favourable conditions for the bacteria to thrive and multiply because the water is often stagnant and maintained at a higher temperature, which is the perfect atmosphere for bacteria to grow. Some examples are listed below: 

  • Cooling towers
  • Shower faucets and taps that are seldom used
  • Hot tubs
  • Fountains and water features
  • Hot water tanks
  • Air conditioning units

If you breathe in tiny droplets of water containing the bacteria, you can get Legionnaires’ disease. It’s unlikely to become ill from drinking contaminated water, but it is possible if the bacteria enter your lungs whilst drinking (aspiration). 

Recent research uncovered that vehicle operators such as truck drivers could potentially be exposed to Legionella bacteria from adding water to windscreen wiper fluid. The bacteria is more likely to thrive in the mixture of water and windscreen wiper fluid than in the fluid alone. Being enclosed in a small, warm tank means that if present, the bacteria could easily replicate. Drivers could then inhale this bacteria when activating the system to clean their windscreens.4 This potentially poses a small risk to all vehicle users, although the study only investigated commercial truck drivers. 

Legionella longbeachae bacteria is another type of Legionella bacteria. Although it is less common than Legionella pneumophila, it is endemic in Australia and New Zealand. Exposure to Legionella longbeachae is linked to potting soil and compost rather than water.5

Signs and symptoms of legionnaires' disease

According to the NHS, symptoms of Legionnaires’ disease are similar to other types of pneumonia:

  • Difficulty breathing
  • A cough
  • Chest pain
  • High temperature/fever
  • Flu-like illness, including body aches
  • Headaches

The CDC states that symptoms usually occur within 2-14 days of exposure to Legionella bacteria, but can develop later. 

Pontiac fever symptoms are generally much milder and more short-lived, occurring within 3 days of exposure. Symptoms include: 

  • Fever
  • Muscle aches

Management and treatment for legionnaires' disease

Antibiotics are required to treat Legionnaires’ disease, which are effective in most cases. They are usually given intravenously. Due to the severity of the pneumonia, assistance with breathing such as oxygen might also be needed. 

Pontiac fever doesn’t require treatment, and patients get better without medical intervention. 

Diagnosis of legionnaires' disease

To determine if a patients’ pneumonia is caused by Legionella bacteria, a urine test and/or a sample of phlegm is taken to be cultured in a lab. 

The urine test can detect the bacteria very quickly, but so far, it can only detect the most common bacteria that causes Legionnaires’ disease, Legionella pneumophila. The cultured specimen results take longer, but can detect other types of Legionella bacteria. 

Pontiac fever can be diagnosed through a blood or urine test. 

Prevention of legionnaires' disease

Reducing the spread of Legionella bacteria is essential to prevent legionellosis. Hotels and hospitals should follow a water management programme to minimise the growth of waterborne bacteria in water systems to protect public health. Even if you own a hot tub or have water fountains in your garden, it’s still important to take steps to prevent potentially harmful bacteria. 

Generally it is important to:6 

  • Maintain water at a temperature that is not warm enough for bacteria to grow
  • Ensure adequate disinfection of water storage areas and tanks 
  • Prevent water lying stagnant
  • Maintain equipment to prevent the buildup of sediment and corrosion 

If you manage/own a building or are an employer, it is your responsibility to manage Legionella growth risks.7

If you are visiting somewhere with water that may not have been used for a while, or have recently moved to a building that has been vacant, Legionella bacteria might be present. To flush out any bacteria, you should run the taps/shower for at least 5 minutes and avoid inhaling any water droplets.8

Complications of legionnaires' disease

Around 1 in 10 cases of Legionnaires’ disease results in death. This number is higher if the disease was acquired in a healthcare setting.9

The likelihood of developing complications increases if the disease is left untreated. These include respiratory failure, shock and multi-organ failure.10


Is legionnaires' disease contagious?

Legionellosis is not thought to be contagious, so it doesn’t spread person-person like some other respiratory conditions. However, a cluster of cases was identified in Portugal in 2014, which likely involved person-to person transmission of Legionnaires’ disease. It is thought that the disease was passed on from a patient to the his mother who was caring for him in very close proximity.11

Who is at risk of legionnaires’ disease?

Most healthy people that come into contact with Legionella bacteria do not become ill. 

However, you are more likely to develop the disease if you have certain risk factors. These include:12

  • Being over 50 years old
  • Being a smoker, or having smoked in the past 
  • Having cancer
  • Having an underlying illness, including diabetes, kidney failure or liver failure
  • Having a weakened immune system due to immunosuppressant drugs or conditions such as HIV
  • Having a chronic lung disease such as COPD 

How common is legionnaires' disease?

In the UK, there are an estimated 4,000 - 6,000 cases each year, but researchers believe that cases largely go unreported because the pneumonia is presumed to be caused by something else. In 2019, there were 503 confirmed cases in the UK.13

When should I see a doctor?

If you have any symptoms of legionellosis, you should consult your doctor. You should definitely mention if you have recently been away from home and used facilities that might not have been used in a while, e.g. hotel showers. Additionally, mention if you have been in a hot tub recently or think that there’s any chance you could have come into contact with Legionella bacteria. This is especially important if you are at higher risk.  


Legionella bacteria doesn’t pose risk to human health when present in many natural water sources such as rivers. However, when it occupies man made water systems, such as hot tubs or a cooling tower, inhaling small water droplets can result in a serious pneumonia called Legionnaires’ disease. 

Symptoms include a cough and difficulty breathing. Patients require hospitalisation to receive antibiotics and assistance with breathing if needed, and usually make a full recovery. Unfortunately, fatalities do occur. Being educated about Legionella can help you take steps to reduce the risk of contracting Legionnaires’ disease. 


  1. Legionellosis (Legionnaires’ disease & pontiac fever) - chapter 4 - 2020 yellow book | travelers’ health [Internet]. CDC. [cited 2023 Mar 31]. Available from: https://wwwnc.cdc.gov/travel/yellowbook/2020/travel-related-infectious-diseases/legionellosis-legionnaires-disease-and-pontiac-fever#:~:text=Legionnaires%27%20disease%20accounts%20for%20nearly,but%20no%20signs%20of%20pneumonia
  2. Chaudhry R, Sreenath K, Agrawal SK, Valavane A. Legionella and Legionnaires’ disease: time to explore in India. Indian J Med Microbiol. 2018;36(3):324–33. 
  3. Gram-negative bacteria infections in healthcare settings [Internet]. CDC. 2019 [cited 2023 Mar 31]. Available from: https://www.cdc.gov/hai/organisms/gram-negative-bacteria.html 
  4. Politi J, Queralt A, Valero N, Martín-Gómez MT, Durán RG, Parra E, et al. Vehicle windshield wiper fluid as potential source of sporadic legionnaires’ disease in commercial truck drivers - volume 28, number 4—april 2022 - emerging infectious diseases journal - cdc. [cited 2023 Mar 31]; Available from: https://wwwnc.cdc.gov/eid/article/28/4/21-0814_article 
  5. Bell H, Chintalapati S, Patel P, Halim A, Kithas A, Schmalzle SA. Legionella longbeachae pneumonia: Case report and review of reported cases in non-endemic countries. IDCases [Internet]. 2021 Jan 14 [cited 2023 Mar 31];23:e01050. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7817369/ 
  6. Legionella water management programs overview [Internet]. CDC. 2023 [cited 2023 Mar 31]. Available from: https://www.cdc.gov/legionella/wmp/overview.html 
  7. Legionnaires’ disease - Are there Legionella risks in my workplace? [Internet]. HSE [cited 2023 Mar 31]. Available from: https://www.hse.gov.uk/legionnaires/workplace-risks.htm 
  8. Legionella [Internet]. EMH Homes [cited 2023 Mar 31]. Available from: https://www.emh.co.uk/housing/for-customers/safety-at-home/legionella/#:~:text=When%20you%20first%20move%20into,and%20showerhead%20for%20two%20minutes
  9. Soda EA. Vital signs: health care–associated legionnaires’ disease surveillance data from 20 states and a large metropolitan area — United States, 2015. MMWR Morb Mortal Wkly Rep [Internet]. 2017 [cited 2023 Mar 31];66. Available from: https://www.facebook.com/CDCMMWR 
  10. Legionellosis [Internet]. WHO [cited 2023 Mar 31]. Available from: https://www.who.int/news-room/fact-sheets/detail/legionellosis 
  11. Correia AM, Ferreira JS, Borges V, Nunes A, Gomes B, Capucho R, et al. Probable person-to-person transmission of legionnaires’ disease. N Engl J Med [Internet]. 2016 Feb 4 [cited 2023 Mar 31];374(5):497–8. Available from: http://www.nejm.org/doi/10.1056/NEJMc1505356 
  12. Legionnaires disease cause and spread [Internet]. CDC. 2023 [cited 2023 Mar 31]. Available from: https://www.cdc.gov/legionella/about/causes-transmission.html 
  13. Legionella risks [Internet]. British Safety Council [cited 2023 Mar 31]. Available from: https://www.britsafe.org/publications/safety-management-magazine/safety-management-magazine/2020/legionella-risks-emerging-from-lockdown/
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Jessica Gibson

Bachelor of Science- BSc(Hons)- Health Sciences- The Open University

Jessica is a Health Sciences graduate with a passion for both Science and English and is delighted to have found a way to combine the two. She is a motivated and enthusiastic writer determined to make scientific information more widely accessible.
Jessica is especially interested in infectious diseases, neurodegenerative diseases, the impact of trauma on physical health, health equity and the health of children residing in developing nations.

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