The people most likely to fall seriously ill from Legionnaires’ disease are older adults, smokers, heavy drinkers, and anyone whose immune system or lungs are already weakened. Healthy young people are exposed to the same water and the same bacteria, but their odds of developing pneumonia from it are far lower [1].

That single fact reshapes your duty. If you run a care home, a hospital ward, an extra-care scheme or a dialysis unit, the bacteria do not have to reach many people to cause harm. They have to reach the wrong person once. The susceptibility is concentrated in exactly the population you serve.

So the question “who is most at risk?” is not academic for a care or healthcare manager. It is the reason a setting like yours cannot run the same minimal water regime that might pass without incident in a low-occupancy office.

The factors that raise the odds

Susceptibility is a stack of factors, and most vulnerable people carry more than one. The NHS lists the recognised influences on who develops Legionnaires’ disease after exposure [1], and they map onto your resident and patient population with uncomfortable precision.

Age. Risk climbs with age, and most confirmed cases occur in older adults [1]. A residential care population sits squarely inside the band where the disease is most often diagnosed.

Smoking and heavy drinking. Smoking damages the lung’s ability to clear inhaled bacteria, and both smoking and heavy alcohol use are associated with higher susceptibility [1]. Plenty of older residents are current or long-term ex-smokers.

A weakened immune system. This is the big one. People on immunosuppressive medication, those having chemotherapy, transplant recipients and people with conditions that suppress immunity are markedly more vulnerable [2]. Healthcare premises concentrate these patients deliberately, which is why dedicated guidance like HTM 04-01 exists for safe water in hospitals [3].

Chronic lung, kidney and other underlying disease. Existing respiratory or kidney conditions, diabetes and similar long-term illness raise the chance that exposure turns into serious infection [1].

Men are diagnosed more often than women, and the disease is uncommon in children [2]. None of this is a reason to relax anywhere, but it tells you where the consequences land hardest.

Why high susceptibility forces rigorous control

Here is the logic that should sit at the front of your water safety thinking. Control measures reduce the probability that viable bacteria reach a person in a breathable aerosol. Susceptibility governs the severity of what happens if that probability is not driven low enough. When your building is full of high-susceptibility people, you have lost the cushion that a healthier population provides. The control regime has to carry the whole load.

That is precisely why the risk-system thinking in HSG274 and the duty to manage under ACoP L8 expect proportionate, written, site-specific control rather than a token annual check [4][5]. In a care or healthcare setting, “proportionate” points one way: toward more frequent monitoring, tighter temperature discipline at outlets used by frail residents, attention to showers and other aerosol sources, and a genuine plan for assets like calorifiers, TMVs and long pipe runs that can harbour the bacteria.

In my view, the most useful mental shift for a manager is to stop thinking of compliance as a building requirement and start thinking of it as a clinical-adjacent safeguard. The water system is part of the care environment. A blended outlet running lukewarm because a TMV is failing is not just a maintenance ticket on a vulnerable ward. It is a growth opportunity sitting in front of the people least able to survive the consequence.

The thing beginners get wrong

The common mistake is to read “older and immunocompromised people are most at risk” and conclude that the control should focus on the rooms those people occupy. It should not. Bacteria multiply wherever water sits warm and stagnant, often far from the vulnerable wing, and travel through the system to an outlet near someone susceptible. A little-used guest bathroom, a decommissioned wing brought back into use, a dead leg behind a removed basin, the bottom of an oversized calorifier, these are where the colony grows.

So the susceptibility of your population justifies rigour across the whole system, not just the obvious clinical areas. Susceptibility tells you the stakes. The risk assessment tells you where the bacteria actually are. You need both. A short, well-illustrated comparison of who suffers most versus where control matters most is the difference between a plan that looks complete and one that is.

A note on what this guidance is and is not

This explains why certain groups are more susceptible so you can pitch your exposure controls correctly. It is not a tool for medically ranking or triaging residents, and it is not clinical advice. Who is “high risk” in a care sense is a matter for clinicians; what you control, and how rigorously, is set by a competent, site-specific Legionella risk assessment carried out under BS 8580-1 and acted on through HSG274 [6][4]. Do not use the susceptibility list to decide some outlets matter and others do not. Use it to justify treating the whole system seriously.

What to do first

Pull your current risk assessment and ask one question of it: does it reflect that this building houses a highly susceptible population, or does it read like a generic template? Look for evidence that monitoring frequency, the asset register and the control scheme were set with your residents’ vulnerability in mind, not a default. If they were not, that is the conversation to have with your responsible person or service provider this week.

If your temperature and flushing records still live in a paper logbook or a shared spreadsheet, that is also the moment to question whether you can actually prove control on demand. In a setting full of vulnerable people, the ability to show an auditor or investigator a complete, time-stamped record of every check, with the gaps flagged before they become incidents, is worth more than a folder nobody has opened since the last inspection. Moving those records into a digital logbook is one practical step that makes rigour visible rather than assumed.

FAQ

Are children at risk of Legionnaires’ disease?

It is uncommon in children, and most confirmed cases are in older adults [2]. That does not make a nursery or a paediatric setting exempt from control, because immunocompromised children and shared water systems still warrant proper management, but the typical case is an older person with one or more underlying risk factors.

Does being healthy mean you are safe from Legionnaires’ disease?

No. Healthy people are exposed to the same aerosols and can still develop the disease; their odds of serious illness are simply lower than those of a susceptible person [1]. Control measures protect everyone in the building, which is why they are not optional even in lower-occupancy settings.

Why are care homes and hospitals treated as higher-risk for Legionella?

Because they concentrate the very groups most likely to develop severe disease, older, frailer, and often immunosuppressed people, in one place served by one water system [2][3]. The susceptibility of the population, combined with aerosol sources like showers, is what pushes these settings toward more rigorous, more frequent control.

Sources

[1] NHS, “Legionnaires’ disease”. https://www.nhs.uk/conditions/legionnaires-disease/ [2] CDC, “About Legionnaires’ Disease”. https://www.cdc.gov/legionella/about/index.html [3] NHS England, “Health Technical Memorandum 04-01: Safe water in healthcare premises”. https://www.england.nhs.uk/publication/safe-water-in-healthcare-premises-htm-04-01/ [4] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [5] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - ACoP and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [6] BSI, “BS 8580-1:2019 - Risk assessments for Legionella control. Code of practice”. https://knowledge.bsigroup.com/products/water-quality-risk-assessments-for-legionella-control-code-of-practice-1