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Knowledge Base » Health and Safety » Legionnaires Disease Symptoms and Treatment

Legionnaires Disease Symptoms and Treatment

Last updated on 20th December 2023

Legionella pneumophila

Legionnaires Disease is a potentially fatal illness affecting approximately 200-250 people in the UK each year. Although this condition is relatively uncommon, outbreaks often involve hundreds of people who have come into contact with the same source.

Most cases of Legionnaires disease can be treated using strong antibiotics. However, the majority of individuals that contract this disease require admittance to a hospital while they undergo treatment.

In this guide, we’ll discuss precisely what Legionnaires disease is, how people contract the illness, and who is at higher risk of developing severe pneumonia if they catch Legionnaires. We’ll also provide information about Legionnaires disease symptoms, onset period, treatment, and recovery.

Next, we’ll look into what measures you can take to prevent Legionella bacteria in your home, before ending this piece with some frequently asked questions and answers.

Although this disease is quite rare, there was a 43% increase in confirmed Legionella cases between 2015 and 2018 in England and Wales.

What is Legionnaires disease?

Legionnaires disease is a severe form of pneumonia caused by exposure to Legionella bacteria. The illness was named after members of the American Legion developed the disease in 1976 as they attended a convention in a Philadelphia hotel.

Legionella bacteria multiply and thrive in warmed water systems that are left idle for extended periods. For this reason, hospitals, hotels, schools, office blocks, and other large buildings are classed as high-risk for Legionella infection.

Legionnaires disease starts to develop after people inhale tiny water droplets that have been infected with the Legionella bacteria. Over the coming days, symptoms develop into potentially fatal pneumonia if the condition is left untreated.

The Legionella bacteria colonies can grow inside man-made water systems. Therefore, risks must be controlled by introducing preventative measures and appropriate testing. Although everyone is susceptible to contracting Legionnaires, it’s particularly dangerous for the elderly, people with weak immune systems, smokers, and those who drink heavily.

As well as placing people at risk of pneumonia, Legionnaires can also cause life-threatening conditions such as organ failure or septic shock.

How do people contract Legionnaires disease?

Bacteria called ‘Legionella Pneumophilia’ causes the majority of cases of Legionnaires disease. This bacteria can survive in soil and water outdoors but rarely causes infections. However, man-made water systems such as air conditioning units and humidifiers allow perfect conditions for Legionella to thrive.

Home plumbing can also provide a breeding ground for Legionella bacteria. Although most outbreaks typically occur in large buildings. This is because larger buildings have more complex systems, making it easier for bacteria to grow and spread. Additionally, air conditioning units found in homes and cars tend to use refrigerant chemicals instead of water for cooling.

Landlord conducting a risk assessment for Legionella

How does legionella infection spread?

The majority of people become infected after they inhale microscopic droplets of water that contain the legionella bacteria. The droplets could originate from a showerhead, tap, jacuzzi, swimming pool, or the air conditioning system in a large hotel.

Almost 50% of all Legionnaires cases diagnosed each year in the UK are linked to travel abroad. When we travel abroad, many of the hotels in popular destinations open seasonally. Therefore, as water has laid dormant between seasons, legionella bacteria has had ample time to thrive.

Legionnaires outbreaks have been linked to:

  • Hot tubs and whirlpools
  • Cooling towers in air conditioning systems
  • Hot water tanks and heaters
  • Decorative fountains
  • Swimming pools
  • Birthing pools
  • Drinking water.

There are several other ways that this infection can be transmitted, including:

  • Soil – Although relatively uncommon, several people have contracted Legionnaires after being exposed to contaminated potting soil or working in a garden.
  • Aspiration – Aspiration is the process where liquid enters the lungs accidentally. This can happen when you choke or cough when drinking or swimming. If the water that you aspirate contains legionella bacteria, you could develop Legionnaires disease.

Note that it’s very uncommon for someone to catch legionnaires from another person diagnosed with the infection or from drinking contaminated water.

Who is at higher risk of developing severe pneumonia from legionnaires disease?

Although anyone can contract Legionnaires disease, people with the following conditions are at increased risk of developing pneumonia and other severe complications:

  • Current or former smokers
  • People over 50 years of age
  • People diagnosed with chronic lung diseases such as COPD or emphysema
  • People with cancer
  • People with underlying illnesses such as liver failure, kidney failure, or diabetes
  • People who have been diagnosed with weakened immune systems or who are currently taking medications that can weaken the immune system
  • People who are dependent on alcohol or drugs.

What are the symptoms of Legionnaires disease?

After exposure to legionella bacteria, Legionnaires disease starts to develop in the next 2-10 days.

Signs of infection often begin with:

  • Muscle aches
  • Headache
  • A temperature that is 104 F or higher.

By the second or third day after legionella exposure, other symptoms that develop include:

  • Chest pain
  • Shortness of breath
  • A persistent cough that could bring up mucus and blood
  • Confusion, or changes in mood
  • Gastrointestinal problems, such as nausea, sickness, and diarrhoea.

Although Legionnaires disease predominantly affects the lungs, it can lead to infections in cuts and wounds or other parts of the body.

There is a mild form of Legionnaires called ‘Pontiac fever’ which can cause muscle aches, chills, and high temperature. However, this condition will to infect your lungs and often clears within a few days.

Woman suffering with Legionella symptoms

When should you seek medical attention?

If you are concerned that you’ve been exposed to legionella bacteria, seek medical attention immediately. The sooner Legionnaires disease is diagnosed and treated, the shorter the recovery period. Additionally, you are less likely to develop severe complications if the illness is caught and treated in its early stages. For people classed as ‘high risk’, prompt medical treatment is essential.

Untreated Legionnaires disease tends to worsen during the first week. The most severe complications of the condition are respiratory failure, acute kidney failure, shock, and multi-organ failure.

Recovery requires urgent antibiotic treatment, which is usually completed after several weeks. In sporadic cases, severe progressive pneumonia can lead to brain sequelae.

The mortality rate for Legionnaires disease depends on:

  • The severity of the disease
  • The appropriateness of initial treatment
  • The setting where Legionella was contracted
  • Host factors of the infected individual.

In untreated Immuno-suppressed patients, death rates can be as high as 40-80%. However, this rate can be reduced to 5-30% via appropriate case management and depending on how severe the clinical symptoms appear. The overall death rate for Legionnaires disease usually falls within the range of 5-10%.

What is the onset period?

The incubation period (the time interval between infection and the appearance of the first symptoms of Legionnaires) is typically cited as being between 2-10 days. Although, in rarer cases, this period can extend to over ten days.


People with suspected Legionnaires disease can have the condition confirmed via chest x-ray. Typically, clinicians use two tests to verify whether or not a patient suffering from pneumonia is infected with Legionella.

  • Urine test
  • A laboratory test that takes a sample of sputum (phlegm) or washing from the lung.

Treatment and complications

Legionnaires disease must be treated using a course of strong antibiotics, and most cases of this condition are successfully treated in the UK. For those who develop more severe symptoms, oxygen and machine-ventilation may be required to help them to breathe.

Healthy people often recover reasonably quickly after being diagnosed with Legionnaires disease. However, to recover, patients will require hospitalisation. Although most people make a full recovery from the condition, it can take several weeks for patients to feel like they are back to normal.

Possible complications of Legionnaires’ disease include:

  • Lung failure
  • Death.

Roughly one in ten people that contracts Legionnaires disease will die due to severe complications brought on by this illness. Statistically, approximately one in four people who contract Legionnaires when staying in a healthcare facility will die.

How to prevent Legionnaires disease

Currently, there are no vaccinations available to prevent people from contracting Legionnaires disease. The key to preventing the growth of legionella bacteria is to endure that landlords and building managers maintain water systems to the highest possible standard.

Examples of residential building water systems that may grow and spread Legionella include:

  • Hot tubs
  • Hot water tanks and heaters
  • Large plumbing systems
  • Cooling towers (structures that contain water and a fan as part of centralised air cooling systems for building or industrial processes)
  • Decorative fountains.

The Center for Disease Control has developed a toolkit to assist building owners and managers with developing and implementing a water management program. These guidelines help to reduce their building’s risk for growing and spreading Legionella.

Legionella and jacuzzis/hot tubs

Legionella growth thrives in warm water, like the temperatures used in hot tubs. However, warm temperatures also make it hard to keep disinfectants, such as chlorine, at the levels needed to kill germs like Legionella. Both disinfectant and other chemical levels in hot tubs must be checked regularly. Additionally, hot tubs must be cleaned as recommended by the manufacturer.

Legionella testing in hot tub

Frequently asked questions

After recovering from Legionnaires disease, can I be re-infected?

Research suggests that once antibiotics are used to cure Legionnaires disease, the bacteria are cleared from the body. It appears that the vast majority of people develop immunity after recovering from their first infection. Therefore, if you’ve had the disease once, it would appear that you have residual protection against further infections. That said, if you smoke, you are still more likely to contract the disease compared to non-smokers. To further prevent re-infection, it’s recommended that you quit smoking.

How common is Legionnaires disease?

The condition is falsely perceived as rare because the public rarely hears about instances when individual cases are detected. However according to Bupa About 300 to 500 people in England and Wales get it each year.

Occasionally, multi-case outbreaks make the national news. However, due to the level of testing required to identify Legionnaires infections, many cases may go undetected and unreported across the world. Some hospitals recognise cases of Legionnaires once they test patients suffering from pneumonia. However, with additional surveillance and testing, more cases are likely to be revealed.

Because the symptoms of pneumonia and Legionnaires disease are almost identical, it’s believed that many undetected cases of the condition are labelled as pneumonia. Many cases may have been prevented if they were identified as Legionnaires, and the source of Legionella bacteria was determined.

What are the long-term side effects of Legionnaires disease?

Once Legionnaires disease has been successfully diagnosed and treated, the survivor may be affected for several months or years with minor side effects. In 2002, a study of 122 Legionnaires survivors in the Netherlands revealed:

  • 75% of patients suffered from fatigue
  • 63% of patients suffered neuromuscular symptoms
  • 66% of patients showed neurological symptoms
  • 15% of patients suffered from PTSD
  • The average time that a patient believed their quality of life was impacted post-recovery was 1.5 years.

As with most acute illnesses, patients who recover from Legionnaires’ disease could suffer from long term side effects. The most common of these are fatigue and lack of energy for several months after the disease is cured.

We hope that you found our guide to Legionnaires disease insightful. If you notice any of the following symptoms and think you may have contracted this condition, we recommend that you see a doctor or call 111 straight away.

Severe symptoms include –

  • You cannot breathe properly
  • You have severe chest pain
  • You have a high temperature or feel hot and shivery
  • You feel like you have severe flu.

The sooner you receive a course of antibiotics or hospitalisation, the higher the chances of making a full and speedy recovery. Legionnaires disease can prove fatal. If you are a landlord or business owner that deals with large plumbing systems that have been dormant for some time, it’s essential that these areas are inspected and sanitised efficiently to prevent the growth of Legionella, you should have up to date training. More information on the UK Legionella guidelines can be accessed with the HSE government website.

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About the author

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Marcel Deer

Marcel qualified as a journalist from Liverpool John Moore's University in 2009. After working in PR and digital marketing for five years, he spent two years working as a social media consultant. Since then, he's worked from 15 countries as a remote content writing/marketing expert.

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