What is the difference between legionella and legionnaires' disease?
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Key points
- Legionella is the bacterium; legionnaires' disease is the illness, a pneumonia caused by inhaling contaminated water droplets.
- Legionellosis is the umbrella term for every illness the bacteria cause, including the milder Pontiac fever and Lochgoilhead fever.
- Infection comes from breathing in aerosol, not from drinking water, and it does not pass from person to person.
- Control is about the water system: keep water out of the 20°C to 45°C growth range, keep it moving, and keep it clean.
- This site does not give medical advice. If you feel unwell, contact NHS 111 or your GP.
Legionella is a bacterium that lives in water; legionnaires' disease is the serious pneumonia it can cause when contaminated droplets are breathed in. The bacteria also cause milder flu-like illnesses, Pontiac fever and Lochgoilhead fever. Together these illnesses are called legionellosis.
The two words are used interchangeably in everyday speech, which causes real confusion: people search for legionella when they mean the disease, and duty holders read disease surveillance figures as if they describe their water system. Keeping them separate makes both topics clearer, so this page sets out the relationship between the organism and the illnesses, and then points back to what this site actually covers, the water systems.
Legionella: the bacterium
Legionella is a family of bacteria found naturally in rivers, lakes and soil water, usually in numbers too low to matter. What is legionella covers the organism in detail. The problems start when the bacteria colonise a man-made water system and find the conditions to multiply: water between roughly 20°C and 45°C, stagnation, and the nutrients in scale, corrosion and biofilm. Of the many species, Legionella pneumophila causes the great majority of legionnaires' disease cases, which is why the figures on this site and in UK surveillance concentrate on it.
Legionnaires' disease: the serious illness
Legionnaires' disease is a lung infection, a form of pneumonia, caught by inhaling fine water droplets carrying the bacteria. It takes its name from the 1976 outbreak at an American Legion convention in Philadelphia, after which the previously unknown bacterium was identified (Fraser and colleagues, New England Journal of Medicine, 1977). It is uncommon but serious: UKHSA surveillance for England and Wales recorded 472 legionellosis cases in 2024, with 13 deaths. Most cases are single, sporadic infections rather than part of an outbreak, and the largest single exposure category in the 2024 figures was community-acquired infection.
Because symptoms, diagnosis and treatment are clinical matters, this site deliberately does not cover them. If you are worried about your health, contact NHS 111 or your GP; that service exists to triage exactly this kind of concern.
Pontiac fever and Lochgoilhead fever: the milder forms
The same bacteria cause two recognised milder illnesses, both short, flu-like, and without pneumonia:
- Pontiac fever, named after a 1968 outbreak at a health department building in Pontiac, Michigan, from which the causative legionella was later isolated (Kaufmann and colleagues, American Journal of Epidemiology, 1981). A UK example is the 2015 Dundee hotel outbreak: 65 people fell ill, 18 with confirmed Pontiac fever, from a leisure-club spa pool that had not been properly drained, cleaned or disinfected. The operator, BDL Select Operations, was fined £54,000 in 2016. See Pontiac fever for more.
- Lochgoilhead fever, a non-pneumonic legionellosis caused by Legionella micdadei, named after the Lochgoilhead outbreak in Scotland described by Goldberg and colleagues in The Lancet in 1989.
These names survive mainly in textbooks and outbreak literature, but they explain a pattern duty holders see in practice: a cluster of flu-like illness linked to a spa pool or similar aerosol source can be legionellosis even when nobody develops pneumonia.
Legionella is the organism in the water; legionnaires' disease, Pontiac fever and Lochgoilhead fever are the illnesses it can cause. You control the illnesses by controlling the water system.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
Why the distinction matters for duty holders
Nobody can see or smell legionella in a system (it is invisible), and most contaminated systems never cause a case. That combination tempts complacency: no illness, so no problem. The surveillance and enforcement record shows the opposite logic at work. The duties under ACOP L8 exist whether or not anyone has fallen ill, and several of the largest fines in our enforcement tracker were for unmanaged risk with no confirmed case at all.
So the practical reading of the health side is simple: the illnesses are prevented upstream, in the plant room. Temperature control, flushing, cleaning, a written scheme and records are the barrier between the bacterium and the disease. The guides to temperature control, flushing and monitoring are where that work is described.
Sources and honesty notes
Case figures above are from UKHSA, Legionellosis in residents of England and Wales: 2024 (published 20 November 2025). Outbreak and fine details are from the primary sources cited in our enforcement dataset. Historical outbreaks are cited to the journal papers that named them. This page explains terminology and points to guidance; it is not medical advice, and it cannot tell you whether any person or any water system is safe.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
Frequently asked questions
What is the difference between legionella and legionnaires’ disease?
Legionella is the bacterium; legionnaires’ disease is the most serious illness it causes, a form of pneumonia caught by inhaling water droplets containing the bacteria. The same bacteria can also cause milder, non-pneumonic illnesses, Pontiac fever and Lochgoilhead fever. The umbrella term for all of these illnesses is legionellosis.
What is legionellosis?
Legionellosis is the collective term for every illness caused by legionella bacteria. It covers legionnaires’ disease, the pneumonic and potentially fatal form, and the milder non-pneumonic forms such as Pontiac fever and Lochgoilhead fever, which are short, flu-like illnesses that do not involve pneumonia and are not fatal.
What is Pontiac fever?
Pontiac fever is a mild, self-limiting flu-like illness caused by legionella bacteria, named after a 1968 outbreak at a health department building in Pontiac, Michigan. Unlike legionnaires’ disease it does not cause pneumonia, most people recover without treatment in a few days, and it is not fatal. The 2015 Dundee hotel outbreak prosecuted as BDL Select Operations was a Pontiac fever outbreak from a spa pool, in which 65 people fell ill, 18 of them confirmed cases.
What is Lochgoilhead fever?
Lochgoilhead fever is a non-pneumonic form of legionellosis, named after an outbreak at Lochgoilhead in Scotland caused by Legionella micdadei, described by Goldberg and colleagues in The Lancet in 1989. Like Pontiac fever it is a short flu-like illness without pneumonia. The two names record the outbreaks where these milder forms were first characterised.
How do you catch legionnaires’ disease?
By breathing in tiny airborne water droplets (aerosol) that contain legionella bacteria, from sources such as showers, spray taps, spa pools, cooling towers or other misting equipment. It is not spread from person to person, and you cannot catch it by drinking water. The bacteria multiply in man-made water systems between roughly 20°C and 45°C, which is why control focuses on temperature and stagnation.
What should I do if I think I have legionnaires’ disease?
This site explains water systems and legal duties; it does not give medical advice, and nothing here can diagnose or rule out an illness. If you feel unwell, contact NHS 111 or your GP, and mention any possible exposure to water systems, for example a recent stay away from home or a workplace with a cooling tower, so clinicians can consider it. Concerns about a water system itself can be reported to the operator or, for workplaces, to the HSE or local authority environmental health.
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