It often starts like flu. A high temperature, aching muscles, a headache, deep tiredness. Then within a day or two a cough sets in, breathing gets harder, and the chest feels tight or painful. That progression, flu-like illness sliding into a chest infection over a few days, is the pattern worth knowing.

Legionnaires’ disease is a serious lung infection (a pneumonia) caused by breathing in tiny droplets of water that carry Legionella bacteria [1]. You cannot catch it from drinking water, and it does not normally spread from person to person. It comes from inhaled mist, from showers, taps, spa pools, air-conditioning cooling towers and similar sources [2].

If you are reading this because you feel unwell and think you may have been exposed, the short answer is below, and so is the line on when to stop reading and ring someone.

What the early symptoms look like

The first signs are easy to mistake for any winter virus. People commonly report a high fever, chills, muscle aches, headache and a general flattened, exhausted feeling [1]. Some also get a loss of appetite. At this stage there may be no obvious chest involvement at all.

Then it tends to move to the lungs. A cough develops, sometimes dry, sometimes bringing up phlegm or occasionally a little blood. Breathing becomes short. There can be a sharp chest pain when you breathe in. A smaller but recognised group get stomach symptoms instead or as well, diarrhoea, nausea or being sick, and some become confused or unusually drowsy [1].

Two things make Legionnaires’ stand out from a routine cold. It usually gets worse rather than better, and the breathlessness and confusion are out of proportion to a “head cold”. Those are the features clinicians take seriously.

How long after exposure symptoms appear

Symptoms do not arrive the moment you step out of a contaminated shower. There is a gap. The incubation period is commonly described as around 2 to 14 days after exposure, with most people falling ill within the first week or so [3].

That delay is the bit people get wrong. Beginners assume that because they felt fine on holiday, or fine the day they used an unfamiliar building’s water, nothing happened. The illness can land a week or more later, by which time the trip or the building is the last thing on your mind. If you have flu-like symptoms now, think back over the past fortnight, not just yesterday.

This is also why noting dates helps. If you can tell a clinician roughly when you might have breathed in water mist, and where, you give them a useful thread to pull.

Legionnaires’, flu or COVID: telling them apart

You cannot reliably tell these apart at home, and you should not try to. They overlap. What raises the index of suspicion for Legionnaires’ specifically is the combination of a worsening chest infection with possible exposure to an aerosol-generating water source, often alongside being in a higher-risk group [2].

A handful of pointers, none of them diagnostic:

  • The trajectory tends to be downhill over days, with rising breathlessness, rather than a cold that peaks and fades.
  • Confusion, drowsiness or marked diarrhoea alongside a chest infection is a flag worth mentioning to a clinician.
  • A recent stay somewhere with showers, a spa or pool you do not normally use, a hotel, a building reopening after a quiet spell, is relevant context.

The honest position: only a test confirms it, and a urinary antigen test or laboratory analysis of a sputum sample is how clinicians actually identify it [4]. Your job is to notice the pattern and pass it on, not to rule anything in or out yourself.

Who is most at risk

Most healthy people who breathe in Legionella do not become seriously ill. The disease tends to hit harder in some groups: people over about 50, smokers and heavy drinkers, those with chronic lung or kidney conditions, diabetes, and anyone whose immune system is weakened by illness or medication [5]. Men are affected more often than women.

If you are in one of those groups and have the symptoms above, treat the threshold for getting checked as lower, not higher.

When to see a doctor

Here is the part that matters most. Do not sit on worsening chest symptoms.

Contact NHS 111 or your GP promptly if you have a fever with a cough, shortness of breath, or chest pain, especially if it is getting worse rather than better, and most especially if you have any reason to think you breathed in water mist in the past fortnight [1]. Mention that possible exposure explicitly, because it changes how a clinician thinks about your case. Early antibiotic treatment matters with this illness.

Call 999 or go to A&E if breathing becomes severely difficult, lips or face turn blue, or someone becomes very confused, unresponsive or hard to rouse.

In my view the single most useful thing you can do before you call is jot down the basics: when symptoms began, and where and roughly when you might have been exposed to showers, a spa pool, or an unfamiliar building’s hot water. It takes two minutes and saves a confused conversation later.

This page is general information, not a diagnosis and not a substitute for medical advice. Legionnaires’ disease can look like many other infections, and only a qualified clinician using the right tests can confirm or rule it out. If your symptoms worry you, the safe move is always to contact a health professional rather than to talk yourself out of it.

FAQ

Can you catch Legionnaires’ disease from another person?

In almost all cases, no. It is caught by inhaling contaminated water droplets from a source such as a shower, tap, spa pool or cooling tower, not by being near someone who is ill [2]. Person-to-person spread is considered very rare, which is why outbreaks are traced back to a water source rather than to a sick contact.

How soon would I know if I had been exposed?

Not immediately. Symptoms typically begin around 2 to 14 days after breathing in the bacteria, so an illness now could trace back to exposure up to two weeks ago [3]. If you feel unwell, think across that whole window when working out where you might have been exposed.

Is there a test for Legionnaires’ disease?

Yes, but it is done by clinicians, not at home. A urine test for Legionella antigen is common, and a sample of phlegm can be analysed in a laboratory to confirm the bacteria [4]. You cannot self-test, which is another reason to describe your symptoms and possible exposure to a doctor.

Does mild flu after using a building’s water mean I have it?

Not necessarily. A short, mild flu-like illness without pneumonia can be Pontiac fever, a far milder relative that usually clears on its own, and it can equally just be an ordinary virus. The feature that warrants prompt medical contact is a chest infection that is worsening, not a mild ache that is fading. If in doubt, ring 111.

Sources

[1] NHS, “Legionnaires’ disease”. https://www.nhs.uk/conditions/legionnaires-disease/ [2] CDC, “How Legionella Spreads”. https://www.cdc.gov/legionella/causes/index.html [3] CDC, “About Legionnaires’ Disease”. https://www.cdc.gov/legionella/about/index.html [4] CDC, “Laboratory Testing for Legionella”. https://www.cdc.gov/legionella/php/laboratories/index.html [5] UKHSA, “Legionellosis in residents of England and Wales: 2024”. https://www.gov.uk/government/statistics/legionellosis-in-residents-of-england-and-wales-2024/legionellosis-in-residents-of-england-and-wales-2024