--- title: "How Legionella spreads through water systems" source_url: https://legionella.io/articles/how-legionella-spreads-through-water-systems/ canonical_url: https://legionella.io/articles/how-legionella-spreads-through-water-systems/ pillar: "Legionella Basics & Science" summary: "Legionella doesn't spread like a cold. Trace its real route - biofilm, warm still water, then mist breathed in - and see where UK sites can break the chain." primary_keyword: "Legionella transmission" date_published: 2025-05-24 date_reviewed: 2026-06-26 author: "Legionella.io editorial team (REMOTE TECH LTD)" reviewed_against: "HSE L8 and HSG274 guidance" region: "United Kingdom" license: "(c) REMOTE TECH LTD. Quote freely with attribution and a link to source_url." --- # How Legionella spreads through water systems Legionella does not spread the way a cold does. There is no chain of coughing colleagues. Instead it makes one short, mechanical journey: it lives in a film on the inside of a pipe or tank, multiplies when the water around it sits warm and still, gets thrown into the air as a fine mist by a fitting, and is breathed into someone's lungs. Five links, in order. Break any one of them and transmission fails. That is the whole value of understanding the mechanism. Once you can name each link, you stop treating Legionella as an abstract lab hazard and start seeing it as a route you can physically trace through your own building — and interrupt at the cheapest point. ## It starts in the film, not the water The water flowing out of a tap is downstream of the real problem. Legionella's home is biofilm: the slick microbial layer that coats the inside of pipes, tanks, shower hoses and tap aerators. That film shelters the bacteria, and the scale, rust, sludge and organic debris that collect in a neglected system feed them. The CDC sets out the conditions that let Legionella establish and multiply — biofilm, favourable warmth, low disinfectant levels, and water that moves slowly or not at all [1]. This is why a glass of clear water tells you nothing. The reservoir is on the surfaces, invisible from the outlet. You are sampling the runoff, not the source. ## What turns a few cells into an infectious dose Presence is not the same as a problem. A handful of bacteria clinging to a pipe wall rarely makes anyone ill. Amplification does — the system has to let the population grow before any of it reaches a person. Temperature is the master switch. As a general expectation reflected in HSE guidance, Legionella multiplies fastest in roughly the 20–45°C band, sits more or less dormant below about 20°C, and is progressively killed above around 60°C [2]. Most UK buildings are riddled with places that drift into that warm middle: an oversized cold tank in a heated plant room, a long horizontal run where stored hot water cools on its way to a far outlet, a blended-warm supply downstream of a thermostatic mixing valve. Time is the second ingredient. Stagnation removes the two things that keep numbers down — fresh disinfectant arriving with moving water, and the simple flushing-away of cells. A dead leg, a capped-but-not-removed branch, or a guest room nobody has booked for a fortnight all give the population the undisturbed hours it needs. Stagnation is the single most common amplifier in practice, and worth understanding in its own right — see [Neglected water systems: the danger of stagnation](https://legionella.io/articles/neglected-water-systems-the-danger-of-stagnation/). Keeping the water out of the growth band is the other half of the job, covered in [Temperature control basics for Legionella prevention](https://legionella.io/articles/temperature-control-basics-for-legionella-prevention/). ## The step that actually reaches people Amplification still does not infect anyone. The bacteria have to get into the air, in droplets small enough to be drawn deep into the lungs. Drinking contaminated water is not the route; inhaling — or in some cases aspirating — fine respirable droplets is [3]. The NHS describes the disease as a lung infection caught from breathing in tiny droplets of contaminated water, and notes it is not normally passed from person to person [3]. So the dangerous fittings are the ones that aerosolise. HSE flags the engineered systems most likely to create that risk — cooling towers and evaporative condensers, hot and cold water systems with spray outlets, and spa pools, among other aerosol-generating plant [4]. A shower head spraying into a small enclosed cubicle is a near-perfect disseminator. A tap slowly filling a bucket is not. The fitting, not the floor area of the building, decides whether Legionella ever leaves the pipework. ## The transmission chain, drawn out If you sketched this on a whiteboard, it would be five labelled boxes joined by arrows, and the point of the picture is that every arrow is a place to intervene. - **Reservoir.** Biofilm on internal surfaces — tank walls, dead legs, shower hoses, tap aerators. This is where the bacteria persist between everything else. - **Amplifier.** Warm, still water sitting in the 20–45°C band long enough for the population to climb. Low turnover plus available nutrients. The reservoir feeds this; this feeds the next box. - **Disseminator.** The fitting that converts contaminated water into airborne droplets: shower, spray tap, cooling tower drift, spa-pool agitation, sometimes a hose or pressure washer. - **Aerosol plume.** The cloud of respirable droplets and how far it drifts. Inside a cubicle it travels a metre; from a cooling tower it can carry well beyond the building boundary. - **Receptor.** A person who inhales the plume — and whether they are more vulnerable to it, for example older people, smokers, or those with weakened immunity or existing lung conditions [3]. Read left to right, that is exactly how a case happens. Read it as a duty holder and each box is a control: clean the reservoir, chill or heat and move the amplifier, eliminate or maintain the disseminator, redirect or contain the plume, and keep the most vulnerable receptors away from high-risk outlets. You do not have to win at every box. You have to break one arrow reliably and prove it. ## Where the chain actually breaks on site The reservoir and the disseminator are usually not the same place — and that trips people up. A spotlessly maintained cold tank can quietly feed a contaminated, rarely-used shower thirty metres downstream. The outlet that infects someone is not necessarily the dirtiest one in the building; it is the one that both makes mist and has a person standing in front of it. When investigators trace Legionella transmission backwards, they are often following the plume to a disseminator, then the pipework back to an amplifier somewhere else entirely. Two practical consequences fall out of this. First, mixing valves are deceptive: by design they blend hot and cold to a comfortable warm temperature right at the point of use, which is precisely the growth band, right at the disseminator. Second, cooling towers are in a category of their own because the plume travels — community clusters have arisen in people who never set foot in the building the tower served. The chain still applies; the arrows are just longer. ## A caveat worth keeping Treat the five-box chain as a thinking tool, not a verdict on your specific site. Which systems in your building genuinely disseminate, how droplets behave at a given outlet, and the temperatures actually achieved in your pipework are all things a competent survey and risk assessment establish by measurement — not by reading a model off a page. The pathway tells you where to look; your assessment tells you what is true there. ## Common questions ### Can you catch Legionella from drinking tap water? Swallowing contaminated water is not the recognised route to Legionnaires' disease; the bacteria have to be inhaled as fine droplets deep into the lungs, with aspiration a secondary mechanism [3]. That is why aerosol-generating fittings like showers and spray taps matter far more than a kitchen tap used only to fill a kettle. ### Does Legionella spread from person to person? Not in the ordinary sense. The NHS notes the disease is not normally passed between people; it spreads from a water source to a person via inhaled aerosol [3]. Several people falling ill together points to a shared water system, not to contagion. ### If our water tests clean, does that mean it can't spread? No. A sample describes one outlet at one moment, and the reservoir lives in biofilm the sample may never touch. A clean result is useful evidence, but it does not prove the chain is broken — controlling temperature, stagnation and aerosol exposure does that, and the records prove it. ## Related reading - [Neglected water systems: the danger of stagnation](https://legionella.io/articles/neglected-water-systems-the-danger-of-stagnation/) - [Temperature control basics for Legionella prevention](https://legionella.io/articles/temperature-control-basics-for-legionella-prevention/) - [HSG274 guidance explained: practical Legionella control](https://legionella.io/articles/hsg274-guidance-explained-practical-legionella-control/) - [Legionnaires disease and Pontiac fever: key differences](https://legionella.io/articles/legionnaires-disease-and-pontiac-fever-key-differences/) ## Sources [1] CDC, "How Legionella Spreads". https://www.cdc.gov/legionella/causes/index.html [2] HSE, "Legionnaires' disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8)". https://www.hse.gov.uk/pubns/books/l8.htm [3] NHS, "Legionnaires' disease". https://www.nhs.uk/conditions/legionnaires-disease/ [4] HSE, "Systems most likely to create legionella risk". https://www.hse.gov.uk/legionnaires/risk-systems.htm