What Is Legionella?

Legionella is a genus of Gram-negative bacteria found naturally in rivers, lakes and soil. It turns into a hazard when it multiplies in warm, stagnant water inside buildings, then spreads in breathable aerosols that can cause Legionnaires' disease, a serious lung infection.

The word describes the bacteria themselves, not the illness they can trigger. Understanding what Legionella is, what makes it grow and why it sits squarely inside health and safety law is the starting point for anyone responsible for a building's water system.

What Legionella bacteria actually are

Legionella is a genus, a family group, of Gram-negative bacteria. Around 60 species have been identified, but a single species, Legionella pneumophila, is responsible for the large majority of human illness, and within it serogroup 1 accounts for most confirmed cases in the UK.

The bacteria are named after the first recognised outbreak, at a 1976 American Legion convention in Philadelphia. They occur naturally and widely in fresh water and damp soil, usually at numbers far too low to cause any harm. In the natural environment Legionella often lives inside single-celled organisms such as amoebae, which both feed the bacteria and shelter them from disinfection.

So when people search for what is legionella bacteria, the honest answer is that it is a common environmental micro-organism that is only a problem in a specific set of conditions. Those conditions are exactly the ones found in badly managed building water systems, which is why it matters to duty-holders rather than only to microbiologists.

What causes Legionella to multiply in water systems

The bacteria being present is not the same as the bacteria being dangerous. The trigger for what causes legionella to reach harmful numbers is a combination of favourable conditions inside pipework and tanks:

• Temperature. Legionella grows fastest between 20°C and 45°C, with an ideal around body temperature. It stays dormant below 20°C and does not survive well above about 60°C. This is the whole basis of temperature control: keep cold water cold and hot water hot, and you deny the bacteria their growth window.
• Stagnation. Still water in a rarely used outlet, an oversized tank or a redundant length of pipe lets bacteria settle and multiply. Movement and turnover disrupt them.
• Biofilm, scale and sediment. A thin layer of biofilm on pipe walls, along with rust, scale and organic debris, gives Legionella nutrients and a physical refuge from heat and disinfectant.
• A host to hide in. Amoebae and other protozoa living in the biofilm act as incubators, allowing the bacteria to reproduce and protecting them from control measures.

Take those conditions away and numbers fall. That is why every recognised control scheme is built around temperature, movement and cleanliness rather than a single one-off treatment.

HSE ACOP L8 and HSG274 Part 2; HSE 'What Legionnaires' disease is and typical symptoms'

Legionella, Legionnaires' disease and legionellosis: the difference

The single most common mistake, and the one worth correcting first, is treating Legionella and Legionnaires' disease as the same thing. They are not. Legionella is the bacterium. Legionnaires' disease is one possible illness it can cause. Legionellosis is the umbrella term that covers all of them. Getting the legionella meaning right changes how you record and communicate risk.

The table below sets out the terms you will meet in HSE guidance and in a risk assessment.

In practice, a duty-holder is managing the bacterium and the conditions that let it grow. Diagnosis and treatment of any resulting illness sit entirely with the NHS. Keeping those two worlds separate stops you drifting into medical claims you are not qualified, or required, to make.

How Legionella makes people ill

Legionella causes illness through one main route: inhalation of tiny water droplets, or aerosols, that are small enough to reach deep into the lungs. Any equipment that turns water into a fine spray or mist can create that aerosol, which is why showers, spray taps, spa pools, cooling towers and water features are all flagged in a risk assessment.

Two points are widely misunderstood and worth stating plainly. You do not catch it by drinking contaminated water, and it does not spread from person to person. The exposure is environmental, from a system, not from another patient.

Some people are more vulnerable to serious infection than others, including those over 45, smokers, heavy drinkers and people with weakened immunity or existing lung or kidney conditions. This site does not cover symptoms, diagnosis or treatment: for anything health-related, direct people to the NHS or a GP. What matters here is the source of exposure, because that is the part a duty-holder can actually control.

Why Legionella is a duty-holder issue, not just a health one

Because the exposure comes from water systems that someone owns and runs, the law treats Legionella as a workplace risk to be managed, not simply a medical misfortune. The framework in Great Britain rests on the Health and Safety at Work etc. Act 1974 and the Control of Substances Hazardous to Health Regulations, with the detail set out in the Approved Code of Practice ACOP L8 and the technical guidance HSG274 (Parts 1 to 3). Healthcare premises follow the additional standard HTM 04-01.

Anyone in control of premises, an employer, landlord, managing agent or building owner, has duties to identify and assess the risk, put controls in place, appoint a competent responsible person, and keep written records of what has been done. None of this makes a building automatically safe. It demonstrates that the risk has been assessed and is being managed, and it records the temperatures, inspections and remedial actions that keep bacterial numbers down.

That recording duty is where most people start. A structured assessment forces you to list every asset, note where warm or stagnant water could sit, and log the checks you carry out. Our free legionella risk-assessment template gives you that structure in one place, so you are recording your findings and controls as you go rather than reconstructing them later.

Where Legionella hides in an ordinary building

Cooling towers get the headlines, but most premises do not have one. For a typical office, gym, care home, school or rented house, the risk lives in the everyday hot and cold water system. When people ask where is legionella found, the answer is: wherever water sits warm and still.

• Cold water storage tanks and hot water calorifiers, especially if oversized or warm
• Long runs of pipework, and dead legs serving outlets that are rarely used
• Showers and spray taps, which produce exactly the aerosol that carries the bacteria
• Thermostatic mixing valves, which blend water down into the growth range close to the outlet
• Spa pools, hot tubs, humidifiers, sprinkler systems and decorative water features
• Any little-used outlet in a partly occupied or seasonal building

The common thread is water held between 20°C and 45°C without regular movement. Mapping those points, and keeping them either cold, hot or flushed, is the practical core of Legionella control. The bacteria are hard to eliminate entirely, so the realistic goal is to deny them the conditions they need and to record that you are doing so.