Legionnaires’ disease is a serious lung infection caused by breathing in water droplets contaminated with Legionella bacteria. It’s preventable, and for anyone who controls a workplace or rented property, preventing it is a legal duty.
Jump to:
- What is Legionnaires’ disease?
- What causes Legionnaires’ disease?
- What are the symptoms of Legionnaires’ disease?
- Who is most at risk of Legionnaires’ disease?
- Where does Legionella grow in commercial buildings?
- What are your legal duties to prevent Legionnaires’ disease?
- Legionnaires’ disease FAQs
What is Legionnaires’ disease?
Legionnaires’ disease is a severe form of pneumonia caused by Legionella bacteria. It belongs to a group of illnesses called legionellosis. The UK Health Security Agency (UKHSA) describes the group as ranging from milder illnesses, such as Pontiac fever, to Legionnaires’ disease, a type of pneumonia that can be severe and sometimes fatal.
The disease takes its name from a 1976 outbreak at an American Legion convention in Philadelphia. It has been a notifiable disease in England and Wales since April 2010, which means doctors must report suspected cases to public health authorities.
What causes Legionnaires’ disease?
People catch Legionnaires’ disease by breathing in fine droplets of water that contain Legionella bacteria. The bacteria live naturally in rivers, lakes and reservoirs at low levels. They become dangerous when they colonise man-made water systems, which provide ideal conditions for them to grow.
Infection needs three things to happen together:
- Legionella multiplying in a water system: usually because water sits between 20°C and 45°C, stagnates, or contains scale, sludge or biofilm.
- An aerosol: showers, taps, spray heads, cooling towers and spa pools release contaminated droplets into the air.
- Someone breathing in those droplets: people with risk factors such as age or smoking are more likely to fall ill.
Infection comes from inhaling droplets, and drinking contaminated water rarely causes illness. That’s why Legionella control focuses on how water systems are managed. For more on the temperatures involved, read our guide to what temperature kills Legionella.
What are the symptoms of Legionnaires’ disease?
Legionnaires’ disease usually starts like flu, with a high temperature, muscle aches and headaches, then develops into a cough, shortness of breath and chest pain. Symptoms usually appear 2–10 days after exposure, and some people also experience nausea, diarrhoea or confusion.
It’s treated with antibiotics, and some people need hospital treatment. Anyone who develops these symptoms after possible exposure should contact their GP or call NHS 111. For full medical information, see the NHS guide to Legionnaires’ disease.
Who is most at risk of Legionnaires’ disease?
People aged 50 and over, smokers and people with weakened immune systems are most at risk, according to UKHSA. Chronic respiratory conditions, diabetes and kidney or liver disease also raise the risk, although anyone can catch it.
UKHSA’s 2024 annual report shows how concentrated the risk is:
- In 2024, almost two thirds of cases in England and Wales (63.9%) were in people aged 60 or over.
- Men accounted for 68% of cases.
- Three in four cases (76.5%) had at least one known risk factor, and smoking was the most common, present in more than half of cases.
If your building houses or treats vulnerable people, the stakes are much higher. Between 2020 and 2024, 21.5% of healthcare-associated cases were fatal, compared with 6.1% of community cases and 2.5% of cases linked to travel abroad. Care homes, hospitals, GP surgeries and supported housing need the most rigorous controls, and in healthcare settings those controls run alongside HTM 04-01.
Where does Legionella grow in commercial buildings?
Legionella grows wherever water stagnates, sits at the wrong temperature, or builds up scale, sludge and biofilm. In commercial and industrial buildings, the most common sources are:
| Where Legionella grows | Why it’s a risk |
|---|---|
| Hot water calorifiers and cylinders | Cooler zones at the base and sludge build-up allow growth below 60°C |
| Cold water storage tanks (tank cleaning) | Warm plant rooms and roof spaces push stored water above 20°C |
| Dead legs and outlets that are rarely used | Stagnant water lets biofilm form |
| Showers and spray taps | Release fine aerosols and collect scale |
| Cooling towers and evaporative condensers | Warm water and spray drift can spread bacteria over a wide area |
| Spa pools and hydrotherapy pools (hydrotherapy pool sampling) | Warm, agitated water creates aerosols at head height |
| Humidifiers, air washers and water features | Spray or mist droplets that people breathe in |
| Industrial process water and vehicle washes | High-pressure spray in warm working environments |
Acorn’s water hygiene monitoring targets these risk points in hot and cold water systems with scheduled temperature checks, flushing and inspections. Every reading is recorded in ServiceTracker, our digital record system.
What are your legal duties to prevent Legionnaires’ disease?
A dutyholder must assess the risk from Legionella and then prevent or control it. This duty applies to employers, landlords and anyone in control of premises. It comes from the Health and Safety at Work etc. Act 1974, the Control of Substances Hazardous to Health Regulations 2002 (COSHH) and the Management of Health and Safety at Work Regulations 1999, and ACoP L8 sets out how to meet it. In practice, a dutyholder must:
- Assess the risk: carry out a Legionella risk assessment through a competent person, and review it whenever there’s reason to think it’s no longer valid, such as changes to the system or how the building is used.
- Appoint a responsible person: name someone with the authority and competence to manage the controls day to day.
- Prepare a written control scheme: set out how the risk will be prevented or controlled, following HSG 274.
- Implement and monitor the controls: carry out temperature checks, flushing, cleaning and, where required, sampling – see our guide to how often to test for Legionella.
- Keep records: retain records of assessments, checks and remedial work for at least five years.
- Register cooling towers: notify the local authority of any cooling towers and evaporative condensers.
Landlords have the same core duty to assess and control the risk in rented homes – see our Legionella risk assessments for landlords. Failing to meet these duties can lead to enforcement action and prosecution, even if no one falls ill. For the full legal framework, see our page on Legionella regulations and compliance.
Legionnaires’ disease FAQs
These answers cover the questions people ask most often about Legionnaires’ disease, from whether it’s contagious to how many cases occur in the UK each year.
Is Legionnaires’ disease contagious?
No, not in normal circumstances. People catch Legionnaires’ disease by breathing in water droplets contaminated with Legionella bacteria from a shared source, such as a shower, spa pool or cooling tower. That’s why outbreaks are traced back to water systems, not to infected people.
Can you get Legionnaires’ disease from air conditioning?
Only from large systems that use cooling towers or evaporative condensers. These serve large air conditioning and industrial cooling systems and can release contaminated droplets that travel beyond the building itself. Self-contained domestic and office units don’t create contaminated spray in the same way.
Can you tell if water has Legionella?
No. Water containing Legionella looks, smells and tastes normal. The only way to confirm whether the bacteria are present is laboratory analysis of water samples. Temperature checks and monitoring show whether conditions favour growth, but they can’t confirm whether Legionella is actually there.
How many cases of Legionnaires’ disease are there in the UK?
UKHSA’s 2024 report, published in November 2025, recorded 463 confirmed cases of Legionnaires’ disease in England and Wales in 2024, down from 609 in 2023. Scotland and Northern Ireland report their figures separately.
Need help preventing Legionnaires’ disease in your building?
Acorn helps dutyholders across Manchester, Leeds, Liverpool, Sheffield and the wider UK meet their Legionella obligations, with risk assessments, Legionella testing and ongoing monitoring to ACoP L8, HSG 274 and HTM 04-01. As a family-run business established in 1990 and a member of the Legionella Control Association (LCA), we record every visit in ServiceTracker, so your compliance trail is always audit-ready.
Responsible for a care home, GP surgery or supported housing? Book a Legionella risk assessment that accounts for vulnerable occupants, or speak to a qualified engineer on 0161 483 7112.