Yes, more or less. Legionella is a natural part of the environment, and it turns up at low levels in rivers, lakes, soil and the treated mains water that feeds your building. Finding it there is normal. It does not mean your water is contaminated, unsafe to drink, or that anything has gone wrong.
That catches a lot of people out, so it is worth slowing down on. The bacteria being present is not the thing that makes anyone ill. What makes water dangerous is when a building system lets a tiny background population multiply into a large one, and then sprays it into the air as a fine mist you can breathe in. Presence is the starting condition for nearly every water system in the country. Amplification is the part you actually control.
Get that distinction straight and most of the worry on this topic falls away.
Where the bacteria come from in the first place
Legionella is naturally occurring. It lives in freshwater environments — lakes, rivers, streams — and in damp soil, generally at numbers far too low to harm anyone [1]. From there it travels, in very small quantities, into the treated water supply that reaches taps across the country.
So the honest answer to “is Legionella in mains water” is that it is commonly present at low levels, the way many environmental bacteria are [2]. Treatment and the disinfectant residual in the mains keep those numbers down, but “low” is not the same as “zero”, and chasing zero in a public supply is neither realistic nor the point. Where Legionella grows: natural and man-made water systems maps this out: the bacteria start in nature and only become a problem when a man-made system gives them what they need.
If you want the wider picture of the organism itself, What is Legionella? An introduction to the bacteria and disease covers the basics.
Presence is normal — amplification is the hazard
Here is the idea the whole subject turns on.
A handful of Legionella cells arriving in cold, moving, treated water is not a hazard. The numbers are low, the water is cold enough to keep them dormant, and it does not sit still long enough for them to build up [3]. That is the background state of an ordinary, well-run system.
Amplification is what happens when that background population is handed the three things it needs to multiply: warmth, stagnation, and something to feed on. Water sitting between roughly 20°C and 45°C — in a tank, a length of dead pipework, or a shower nobody uses — is in the range where the bacteria grow [4]. Add the sediment and biofilm that collect in still water, and a few harmless cells can become a serious population over weeks [3].
The number of bacteria is everything. The dose that can cause infection is far higher than the trace amount that drifts in from the mains. A system only becomes a real risk once it has amplified that trace into something much larger — and then created a way to breathe it in.
The bit beginners get wrong
The instinctive question — is the Legionella in my tap water going to make me ill? — points at the wrong thing.
You do not catch Legionnaires’ disease by drinking water. The bacteria have to reach deep into your lungs, riding on aerosol: the fine, breathable mist a shower, spray tap or cooling tower throws off [5]. Swallowing water sends it to your stomach, not your lungs. We cover this in full in Can you get Legionnaires’ disease from drinking water?.
So the real question is not “is it present in the water” — it almost always is, at trivial levels — but “is my building amplifying it, and is it being turned into a mist someone breathes in?” Two boxes have to be ticked for harm: a high enough number of bacteria, and a route into the lungs. Background mains water ticks neither.
Myth versus reality
The gap between what people fear and what actually drives risk is wide. Here is the honest version.
| The worry | What’s actually true |
|---|---|
| ”Legionella in my tap water means it’s contaminated” | Low-level presence is normal almost everywhere; it is multiplication in your system that signals a problem [1][2] |
| “If it’s naturally occurring, it can’t be controlled” | You can’t remove it from the environment, but you fully control whether your system lets it grow [3][4] |
| “I should stop drinking the tap water” | Drinking isn’t the infection route; the risk is inhaling aerosol from a contaminated system [5] |
| “Any test that finds Legionella is a failure” | Counts matter — a small detection is read very differently from heavy growth [3] |
| “Bottled water is safer against Legionnaires‘“ | The risk was never in drinking, so bottled water changes nothing about it |
The pattern: presence is the normal background. Numbers, warmth, stagnation and aerosol are what turn that background into a hazard.
What this means for you in practice
If you are a tenant, none of this should stop you using your taps or drinking your water. The sensible habits are small. If a shower or tap has stood unused for a couple of weeks — a spare bathroom, a property you have just moved into — let it run before normal use: the cold to clear standing water, the hot until it is properly hot, ideally with the room ventilated and yourself out of the spray.
If you are a landlord or facilities manager, the takeaway is sharper. Your job is not to sterilise the incoming supply — it is to stop your own system amplifying what the supply already contains. That comes down to a few reliable controls: keep cold water genuinely cold (below 20°C where practicable), keep stored hot water hot, and don’t let water sit still [4]. Stagnation is the single biggest gift you can give the bacteria; Neglected water systems: the danger of stagnation explains why an idle system drifts into the danger range so quickly.
A note on scope
This is general background on how Legionella behaves in water supplies, not an assessment of your particular building. Whether your tanks, pipework and outlets are actually keeping the bacteria in check is a judgement for a competent person working from a site-specific risk assessment, and the right controls for a care home, a holiday let and a small office all differ. Nothing here is legal, medical or design advice.
Common questions
Is Legionella in all tap water?
In practical terms it is commonly present at very low levels in the natural environment and the mains supply, because the bacteria are naturally occurring [1][2]. That low background is not what makes water dangerous. The concern only arises when a building system lets those numbers multiply and then aerosolises the water.
Is tap water in the UK safe to drink?
Yes. UK mains water is treated and monitored, and Legionella is not a reason to stop drinking from the tap. The bacteria don’t cause illness through the stomach — infection comes from breathing in contaminated aerosol, not from swallowing water [5].
If Legionella occurs naturally, why bother controlling it?
Because you are not trying to remove it from nature — you are stopping your own system turning a harmless trace into a hazardous quantity. Temperature control, flow and cleanliness deny the bacteria the warmth and stagnation they need to amplify [3][4]. Control targets the conditions, not the background presence.
Does a positive Legionella test mean my water is unsafe?
Not on its own. The count matters, and so does where the sample came from. A low detection is interpreted very differently from heavy growth, and your risk assessment and a competent advisor should guide what — if anything — needs doing [3]. A single number out of context is not a verdict.
What to do next
Stop worrying about whether Legionella is “in” your water — assume a trace is, the same as nearly every building in the country — and put your attention where it pays off: the warm, still corners of your own system. Spend ten minutes today listing the outlets nobody uses and any tank or pipe run that sits lukewarm. Those are where a harmless background becomes a real population. Flush the unused ones, check the temperatures, and write down what you found — a dated record is what proves the background never got the chance to amplify.
Sources
[1] CDC, “About Legionnaires’ Disease”. https://www.cdc.gov/legionella/about/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] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [4] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [5] CDC, “How Legionella Spreads”. https://www.cdc.gov/legionella/causes/index.html