The basics
What is legionella?
Legionella is a type of bacteria found naturally in rivers and ponds. It becomes a hazard when it grows in man-made water systems and people breathe in fine droplets of that water. The illness it causes, Legionnaires' disease, is a serious form of pneumonia.
Last reviewed October 2026 against HSE guidance.
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Where legionella comes from
HSE explains that legionella bacteria are widespread in natural water such as rivers and ponds, but conditions there are rarely right for people to catch the disease. Outbreaks come from purpose-built systems where water is kept warm enough to encourage growth: cooling towers, evaporative condensers, hot and cold water systems and spa pools.
HSE says all man-made hot and cold water systems are likely to provide an environment where legionella can grow. That is why the duty to assess applies so widely.
The conditions it needs
HSE lists the conditions that increase the risk:
- water temperature in all or part of the system between 20°C and 45°C
- breathable droplets being created and dispersed, for example by a shower or a cooling tower
- water that is stored or recirculated
- deposits that feed the bacteria, such as rust, sludge, scale, organic matter and biofilm
Take away any of these and the risk falls. Control measures therefore focus on temperature, on keeping water moving, and on keeping tanks and outlets clean.
How people catch it
People contract Legionnaires' disease by inhaling small droplets of water suspended in the air that contain the bacteria. Showers, spray taps, spa pools and cooling towers all produce droplets of this kind.
The NHS says you cannot usually get it from drinking water that contains the bacteria, from other people who have the infection, or from places like ponds, lakes and rivers.
Who is most at risk
HSE says everyone is susceptible to infection, and that the risk increases with age. People at higher risk include:
- people over 45
- smokers and heavy drinkers
- people with chronic respiratory or kidney disease
- people with diabetes, lung or heart disease
- anyone with an impaired immune system
This is why a risk assessment asks who uses the building. The same water system is a bigger concern in a care home than in a house shared by healthy adults.
This site covers water systems, not health. If you think you may have Legionnaires' disease, follow NHS advice and contact your GP or NHS 111.
What you are expected to do about it
If you are an employer, control premises or rent out property, you have a duty to assess the risk from legionella and keep it under control. For most homes that means a simple assessment and a few routine habits.
Questions people ask
What is legionella?
Legionella is a bacterium found naturally in water. It can grow in man-made water systems and causes Legionnaires' disease when people breathe in fine droplets of contaminated water.
Where is legionella found?
It occurs naturally in rivers and ponds. The risk to people comes from purpose-built systems such as hot and cold water systems, cooling towers and spa pools, where water can sit at temperatures that encourage growth.
Can you get legionella from drinking water?
The NHS says you cannot usually get Legionnaires' disease from drinking water that contains the bacteria. Infection comes from breathing in small droplets.
Is legionella common in homes?
HSE says the risks from hot and cold water systems in most residential settings are generally considered to be low, because water is used regularly and turns over.
Sources
- HSE: What Legionnaires' disease is and typical symptoms
- HSE: Legionella and landlords' responsibilities
- NHS: Legionnaires' disease
- HSE: Legionnaires' disease, what you must do
Checked against these sources in October 2026. This page is general information, not legal advice.
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