Legionella & Water Hygiene · 8 min read
Updated 28 May 2026
What Is Legionella? Bacteria, Disease and Where It Grows
Legionella is a waterborne bacterium that causes Legionnaires' disease, a severe pneumonia. People catch it by breathing in a fine spray (aerosol) of contaminated water — not by drinking it and not from another person. It thrives in stagnant, lukewarm water between 20°C and 45°C, which is why UK plumbing standards keep water out of that band.
The bacterium itself
Legionella is a genus of around 60 species; the one that causes most disease is Legionella pneumophila. It lives naturally in rivers, lakes and damp soil at low concentrations and is normally harmless there. The trouble starts when it gets into a building's plumbing and finds the conditions to multiply: warmth, stagnation, nutrients (scale, sediment, biofilm) and time.
Temperature ranges that matter
- Below 20°C — dormant, no significant growth.
- 20-45°C — the "danger zone" — multiplies fastest around 35°C.
- 50°C — killed within about an hour.
- 60°C — killed within about two minutes.
- 70°C — killed almost instantly.
How you catch it
Infection happens when contaminated water is broken into droplets small enough to be inhaled deep into the lungs. The classic sources are:
- Showers, especially first use after a vacancy or with a scaled head.
- Spray taps, water features and decorative fountains.
- Cooling towers and evaporative condensers on commercial premises.
- Spa pools and hot tubs with inadequate disinfection.
- Humidifiers, room misters and some respiratory therapy equipment.
You cannot catch Legionnaires' disease from another person, from drinking water, or from casual contact. It is an inhalation disease.
Symptoms and severity
Symptoms usually start 2-10 days after exposure (occasionally up to 19 days). It begins like severe flu — fever, chills, muscle aches, headache, tiredness — and progresses to a persistent cough, shortness of breath, chest pain, sometimes diarrhoea and confusion. It is treatable with the right antibiotics but the earlier it is diagnosed, the better.NHS guidance is to seek prompt medical advice if you have these symptoms and have recently been somewhere with possible aerosol exposure.
A milder, flu-like form caused by the same bacteria — Pontiac fever — is self-limiting and doesn't cause pneumonia, but it's a useful warning that water somewhere is contaminated.
Where it grows in a building
Anywhere water sits warm and still. The classic offenders we see on real jobs:
| Hotspot | Why it's a problem |
|---|---|
| Poorly heated calorifier / cylinder | If stored hot water drops below 60°C, the lower portion sits in the danger zone. |
| Cold tank in a warm loft | Cold supplies can heat up above 20°C in summer — especially in an uninsulated loft. |
| Long dead legs | Stagnant water with no flow — see our blog on dead legs in plumbing. |
| Rarely used outlets | Spare en-suites, guest WCs and outside taps in commercial premises. |
| Scaled showerheads & TMVs | Scale shelters bacteria from biocides and heat; aerosolises on use. |
| Vacant buildings | Stagnation across the whole system — see empty properties guidance. |
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Who is most at risk
- Age 45+ — risk rises with age.
- Smokers and heavy drinkers — already-compromised lungs.
- Chronic disease — lung disease, kidney disease, diabetes, cancer.
- Immunocompromised — transplant recipients, certain medications.
- Men are roughly twice as likely as women to develop the disease.
How we prevent it
Prevention is engineering, not medicine: keep water out of the danger zone (the 20/50/60 rule), keep it moving, keep the system clean (no scale, no biofilm, no debris in tanks), and remove dead legs. That's the entire job. Read the full approach in our Legionella control pillar guide.
Ninja Tip
Frequently Asked Questions
How do you catch Legionnaires' disease?
By inhaling tiny droplets of water (an aerosol) that contain Legionella bacteria — for example from a shower, spray tap, cooling tower, spa pool or contaminated humidifier. It is not caught from drinking water and it does not pass from person to person.
What are the symptoms of Legionnaires' disease?
It looks like a severe flu that becomes pneumonia: high fever, chills, muscle aches, headache, persistent cough, shortness of breath and sometimes confusion, diarrhoea or chest pain. Symptoms typically begin 2-10 days after exposure. Anyone with these symptoms — especially if they've been near aerosol-producing water — should seek medical advice promptly.
Where does Legionella grow in a building?
Anywhere water sits warm and stagnant: poorly heated calorifiers and cylinders, cold water tanks warmed by their surroundings, long dead legs, rarely used outlets, scaled showerheads and TMVs, and unused branches after building works. Sediment, scale and biofilm provide shelter from heat and disinfectants.
Who is most at risk from Legionnaires' disease?
People over 45, smokers and heavy drinkers, people with chronic lung or kidney disease, diabetes, cancer or weakened immune systems. Healthy children rarely develop the disease. Fatality is roughly 10% overall and significantly higher in vulnerable groups.
Sources & further reading
Guidance only. This article is general information for UK readers, not a substitute for a site-specific assessment by a competent person. Regulations and best practice change — always check the current official guidance and, for compliance work (Legionella risk, unvented cylinders, water regulations), use a suitably qualified professional. Pipe Assassin is an electric-boiler and water-hygiene specialist and is not Gas Safe registered; we do not carry out gas work.
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