Legionella does not drift through your pipework as a lone cell waiting to make someone ill. It runs a life cycle — a loop of hiding, feeding, multiplying and dispersing — and that loop only turns when a building hands it the right conditions. See where the loop runs in your system and you stop guessing at controls. You know which link each one is meant to break.
This is the part the basic explainers skip. “Keep hot water hot and cold water cold” is sound advice, but it lands harder once you can picture what the temperature is doing to the organism at each stage.
The loop, stage by stage
Start with where Legionella lives when nobody is looking. It rarely floats free in clean water for long. It settles into biofilm — the slick living film of microorganisms, scale and organic debris that coats the inside of tanks, pipe walls, shower hoses and tap fittings. Biofilm is shelter and larder at once: it traps nutrients, shields the bacteria from disinfectant, and holds the other organisms Legionella depends on.
Because Legionella is, unusually, a parasite of larger microbes. In building water it multiplies mainly inside free-living amoebae and other protozoa that graze on the biofilm. The amoeba engulfs the bacterium expecting a meal; the bacterium survives inside it, hijacks the cell, and replicates until the host bursts and releases a fresh crop back into the water and biofilm [1]. That intracellular phase is the engine of the whole cycle, and it is why scrubbing visible slime matters more than it looks — you are removing the nursery, not just the dirt.
The released cells are now planktonic: free-swimming, mobile, able to colonise new surfaces, settle into fresh biofilm, or be carried towards an outlet. The loop closes when those cells are broken into breathable droplets at a shower, spray tap, cooling tower or similar fitting and inhaled. Drinking the water does not cause infection; breathing the aerosol can [2]. From the building’s point of view, every stage before that aerosol is multiplication, and the aerosol is exposure.
What feeds the loop
The cycle turns fast or slow depending on four conditions, and they are the same four you can act on.
Temperature is the master control. As a general expectation reflected in HSE guidance, Legionella multiplies most readily in roughly the 20–45°C range, sits largely dormant below about 20°C, and is progressively killed above around 60°C [3]. Treat those as guidance figures your risk assessment confirms for your plant, not as a law of physics — but the principle holds: tepid water is the growth zone, and a building makes tepid water wherever it lets hot and cold drift towards each other.
Stagnation is the second. Moving water disturbs biofilm, carries heat to where it belongs, and keeps any disinfectant residual circulating. Still water does the opposite. A dead leg, an oversized cold tank, a room left void for a fortnight, a capped-off branch nobody removed — each is a pocket where the loop runs undisturbed [4].
Nutrients are the third. Scale, sediment, rust and the biofilm itself give the protozoa something to graze and the bacteria somewhere to hide. A clean, well-flushed system starves the cycle; a scaled, silted one feeds it.
Loss of disinfectant residual is the fourth, where a system relies on chemical control. CDC groups biofilm, warm temperatures, low or absent disinfectant and slow or stagnant flow together as the conditions that let Legionella grow in building water [1]. Notice how they tend to arrive together: the same neglected branch that goes stagnant also goes tepid, gathers sediment and loses any residual. Risk compounds.
A picture you can sketch
If you want one mental diagram, draw it as a wheel — four points on the rim, one hub.
The hub is the biofilm on the inside of your pipework. Around the rim, clockwise: (1) amoebae and protozoa graze the biofilm; (2) Legionella is taken up and replicates inside them, then bursts free; (3) free-swimming cells disperse through warm, still water and reseed new biofilm; (4) at an aerosol-producing outlet, some of those cells leave the system as a breathable mist.
Now lay your controls over the same wheel. Cleaning and descaling attack the hub. Temperature control suppresses point (2) — too cold to replicate, or hot enough to kill. Flushing and good turnover break point (3) by never letting water sit. Outlet management — point-of-use filtration, removing redundant sprays, limiting aerosol where vulnerable people are present — guards point (4). A programme that only touches one point of the wheel leaves the loop turning everywhere else, which is the usual reason a “compliant” system still grows Legionella.
Where the loop actually defeats people
Two failure modes account for most real-world growth, and both are about the cycle running where no one is watching.
The first is the intermittently-used outlet: a shower in a rarely-let room, an emergency shower no one tests, a sports-hall changing block over the school holidays. Use elsewhere keeps the main system turning over and masks the problem, so the building “looks” controlled — while one starved branch quietly completes the loop and aerosols it into the first person who finally opens the tap [4].
The second is thermal short-circuiting. Hot and cold pipes share a warm riser, a cold supply picks up heat in an unventilated plant room, or a hot return loses temperature before it reaches a far outlet. None of it shows on a logbook that only records temperatures at the easy, well-used points. The loop runs in the gap between where you measure and where the water actually goes lukewarm.
How sure can you be
The life cycle described here is established microbiology, but how it plays out in your building depends on your pipework, your usage and your control strategy — which is exactly why UK practice puts a competent, site-specific risk assessment at the centre rather than a universal rulebook [5]. The temperature bands above are widely used guidance, not guarantees; confirm the figures and how they apply through that assessment. And remember a single laboratory result tells you about one outlet at one moment, not whether the loop is broken everywhere it runs [6].
Where to look first
Take the wheel for a walk. Pick one rarely-used aerosol outlet and one length of pipe you suspect runs warm, and ask of each: at which point on the loop is this vulnerable, and which of my controls is supposed to break it there? If you cannot name the control, you have found a gap worth writing down before your next risk-assessment review.
For how these growth conditions arise across different system types, see Where Legionella grows; for a closer look at the hub of the wheel, Biofilms covers how the film itself shelters the organism.
FAQ
Does Legionella need amoebae to multiply, or can it grow on its own?
In building water systems it multiplies mainly inside free-living amoebae and protozoa living in biofilm, and that intracellular stage is the main route to the numbers that matter for risk [1]. This is why controlling biofilm and the conditions that sustain it is central, rather than treating the bacterium in isolation.
If I keep water hot and cold, do I still need to worry about the rest of the cycle?
Temperature suppresses the multiplication stage, which is a powerful lever, but it does not remove the biofilm, scale or stagnant branches where the loop still runs wherever water drifts tepid [3]. Cleanliness and turnover cover the points temperature alone misses.
Why can a system test clean and still grow Legionella later?
A sample describes one outlet at one moment. The cycle continues in biofilm and stagnant branches the sample never touched, so a clean result is evidence for the sampled conditions, not proof the loop is broken across the system [6].
Sources
[1] CDC, “How Legionella Spreads”. https://www.cdc.gov/legionella/causes/index.html [2] CDC, “About Legionnaires’ Disease”. https://www.cdc.gov/legionella/about/index.html [3] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [4] HSE, “Systems most likely to create legionella risk”. https://www.hse.gov.uk/legionnaires/risk-systems.htm [5] 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 [6] HSE, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm