Legionnaires Disease Symptoms

Guide to Early Signs, Exposure Risks and Legal Duties

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What Legionnaires’ Disease Is And Its Main Symptoms

Legionnaires’ disease is a potentially serious lung infection, a form of pneumonia, caused by legionella bacteria. The symptoms often start like flu, with a high temperature, headache, muscle aches and tiredness, before developing into a cough, shortness of breath and chest pain. It can also cause confusion, nausea or diarrhoea.

Symptoms usually appear between 2 and 10 days after exposure. People generally catch it by breathing in tiny droplets of contaminated water, not by drinking it, and it is not usually passed from person to person. Because the early signs overlap with flu, COVID-19 and other chest infections, symptoms alone cannot confirm the illness. A diagnosis needs a qualified medical professional, usually supported by specific tests.

This page explains the symptoms and how to recognise them, how and where exposure happens, who is most at risk, and the UK legal duties that sit with employers, landlords and other dutyholders responsible for water systems. It is general information for a UK audience and does not replace medical advice or a professional risk assessment.

What Is Legionnaires’ Disease?

Legionellosis is the collective term for infections caused by legionella bacteria. It covers two main forms: Legionnaires’ disease and Pontiac fever.

Legionnaires’ disease is the serious form. It is a type of pneumonia that affects the lungs and can make people very unwell, particularly those who are older or already in poor health. It can require hospital treatment and, in some cases, it can be fatal, which is why early recognition and prompt medical care matter.

Pontiac fever is much milder. It is a flu-like illness that does not usually cause pneumonia and generally clears up on its own without specific treatment. Because its symptoms are less severe and non-specific, many cases are never formally identified.

Both illnesses are caused by the same group of bacteria, which occur naturally at low levels in rivers, lakes and soil. The problem arises when these bacteria find their way into engineered water systems, such as those in buildings, and multiply to numbers that can cause infection when the water is dispersed as a fine spray or mist.

Early And Later Symptoms To Be Aware Of

The symptoms of Legionnaires’ disease often develop in stages. Recognising the pattern can help people decide when to seek medical advice, but the presence of these symptoms does not confirm the illness, as they are common to many respiratory infections.

Early symptoms are usually flu-like and may include:

a high temperature or fever
headache
muscle aches and pains
tiredness and a general feeling of being unwell

As the infection develops, later or more severe symptoms may include:

a persistent cough, which can sometimes bring up phlegm or, in some cases, blood
shortness of breath
chest pain
confusion or changes in mental state
nausea, vomiting or diarrhoea

A worsening chest infection combined with confusion or digestive symptoms can be a feature of Legionnaires’ disease, but it is not unique to it. Anyone with worsening breathing difficulties, chest pain or confusion should seek medical help promptly rather than waiting to see how symptoms progress.

How Long Symptoms Take To Appear After Exposure

Symptoms usually appear between 2 and 10 days after exposure to contaminated water droplets. The period can occasionally be longer, so a longer gap between a possible exposure and the onset of illness does not rule it out.

This incubation window is one reason why it can be difficult to link an illness to a particular source. Someone may fall ill several days after staying in a hotel, using a spa pool, being treated in hospital, or visiting a building with a complex water system. Because of this delay, it helps to think back over recent activities and locations when speaking to a healthcare professional, as this information can support both clinical assessment and any public health investigation.

When To Seek Medical Help And What To Tell Your Doctor

Seek medical advice if you develop symptoms of a chest infection that are getting worse, particularly a high temperature with a cough, breathlessness or chest pain. Contact your GP or call NHS 111 if you are worried about your symptoms.

Seek urgent medical help, or call 999 in an emergency, if you or someone else has severe difficulty breathing, sudden confusion, chest pain that does not ease, or becomes seriously unwell. Do not ignore severe or rapidly worsening symptoms.

Legionnaires’ disease is diagnosed by a clinician, usually with specific tests such as a urine test or samples from the chest, rather than on symptoms alone. Because early symptoms look like many other infections, giving your doctor a clear picture of where you have been can help.

It is worth mentioning to a healthcare professional if, in the days before becoming unwell, you have:

stayed in a hotel, holiday accommodation or on a cruise
used a spa pool, hot tub or swimming facility
been an inpatient in a hospital or stayed in a care home
visited or worked in a building with showers, cooling towers or other water systems

This exposure history does not confirm a diagnosis, but it can prompt appropriate testing and, where relevant, allow public health teams to look into a possible source.

How People Catch Legionnaires’ Disease

People generally catch Legionnaires’ disease by breathing in small droplets of water, known as aerosols, that contain legionella bacteria. These droplets are fine enough to be inhaled deep into the lungs.

It helps to understand what does not usually cause infection. You do not catch Legionnaires’ disease by drinking contaminated water, and it is not usually spread from person to person, although rare exceptions have been reported. The risk comes from water systems that can create a breathable spray or mist, rather than from ordinary contact with another person.

Aspiration can also play a part in some cases, where contaminated water accidentally enters the lungs, for example in people with swallowing difficulties. For most people, though, the relevant risk is inhaling aerosols from a system where legionella has been allowed to multiply.

Where Legionella Exposure Can Happen

Legionella exposure can occur wherever a water system produces a breathable aerosol and conditions have allowed the bacteria to grow. Common sources include:

showers and taps, particularly where they are used infrequently
hot and cold water systems in buildings
cooling towers and evaporative condensers
spa pools and hot tubs
humidifiers and air washers
indoor decorative fountains and water features

Some settings carry a higher potential for exposure because they have larger or more complex water systems, vulnerable occupants, or equipment that generates aerosols. These can include:

healthcare premises such as hospitals
care homes
hotels and holiday accommodation
schools and other public buildings
workplaces with cooling towers, process water or extensive plumbing

The presence of these systems does not mean exposure will occur. The level of risk depends on how the water is stored and used, the temperatures it is held at, and how well the system is managed and maintained.

Who Is Most At Risk

Anyone can develop Legionnaires’ disease, but some people are more likely to become seriously ill if they are exposed. Recognised risk factors include:

being over 45 years of age
smoking, or being a heavy drinker
chronic respiratory or lung disease
kidney disease
diabetes
a weakened immune system, including through certain treatments or conditions
other serious underlying health conditions

These factors describe groups who are more susceptible to serious illness. They do not predict whether any individual will become infected or how severe an illness will be. When dutyholders assess and manage water systems, the presence of susceptible people, for example in a care home or hospital ward, is an important factor in deciding how tightly the risk needs to be controlled.

UK Legal Duties For Employers And Dutyholders

In Great Britain, employers and those in control of premises have a legal duty to assess and manage the risk of exposure to legionella where a work activity or a water system creates a foreseeable risk. This is not a voluntary matter of good housekeeping; it flows from health and safety law.

The core duties are to:

identify and assess the risk of exposure to legionella
prevent the risk where reasonably practicable, or otherwise control it
put in place and maintain suitable control measures
keep records of what has been done
appoint someone competent to help manage the risk
review the assessment and controls when things change

A common way to demonstrate control is to appoint a competent responsible person to take day-to-day charge of managing the risk, supported by a written scheme of control. The responsible person needs sufficient knowledge, training and experience for the role, whether they are an internal appointee or supported by external specialists.

Responsibility does not always sit neatly with one party. It may depend on who owns the building, who has day-to-day control of the water system, the terms of any lease and repairing obligations, and who manages contractors and holds the records. In a rented property, for example, a landlord may assume responsibility has passed to the tenant, but the lease terms and who actually controls the system may say otherwise. In multi-occupied buildings, control of common parts and demised areas can split the duty between different parties.

Appointing a contractor or consultant can support the dutyholder, but it does not transfer the legal responsibility away from the employer or person in control. The duty remains with the person who has control of the premises or the relevant work activity.

UK Legislation And Guidance That Applies

Several pieces of UK legislation and official guidance apply to legionella risk. It helps to understand which are law and which are guidance explaining how to comply.

Health and Safety at Work etc. Act 1974: places a general duty on employers and the self-employed to protect employees and others, so far as is reasonably practicable, from risks to health and safety, including legionella.
Control of Substances Hazardous to Health Regulations 2002 (COSHH): legionella is treated as a biological agent hazardous to health, so the duties to assess, prevent or control, monitor and review apply.
Management of Health and Safety at Work Regulations 1999: requires a suitable and sufficient risk assessment, meaning one that reflects the actual water systems, the people who may be affected and the level of risk.
Approved Code of Practice L8 (Legionnaires’ disease: The control of legionella bacteria in water systems): sets out how to comply with the law. An Approved Code of Practice has a special legal status. Following it is normally enough to comply, and a court may treat a failure to follow it as evidence of a breach unless compliance was achieved another way.
HSG274: technical guidance supporting L8, in three parts. Part 1 covers evaporative cooling systems, Part 2 covers hot and cold water systems, and Part 3 covers other risk systems such as spa pools, humidifiers and air washers. This is guidance rather than law in itself.
Notification of Cooling Towers and Evaporative Condensers Regulations 1992: require certain wet cooling towers and evaporative condensers to be notified to the local authority.
Reporting of Injuries, Diseases and Dangerous Occurrences Regulations 2013 (RIDDOR): may require reporting where a case of legionellosis can reasonably be attributed to a person’s work with a water system.
Health Technical Memorandum HTM 04-01: water safety guidance for the NHS estate in England, sitting alongside L8 and HSG274.
Health and Social Care Act 2008 (Regulated Activities) Regulations 2014: require CQC-registered providers to provide safe premises, which includes managing water safety in care settings.

The Health and Safety Executive is the lead enforcing authority for legionella risk in most workplaces, while local authority environmental health teams enforce in many lower-risk premises such as some offices, hotels and retail, and receive cooling tower notifications. HTM 04-01 applies to the NHS estate in England, with equivalent guidance in the devolved nations.

These duties apply across England, Wales and Scotland. In Northern Ireland, broadly equivalent health and safety legislation applies and is enforced separately, so duties there should be checked locally. Public health notification arrangements also differ between the UK nations, with UKHSA acting as the public health body in England.

Water System Conditions That Increase The Risk

Legionella bacteria are most likely to multiply to harmful levels when several conditions come together in a water system. Understanding these conditions helps explain why control measures focus on the things they do.

The key risk conditions are:

Temperature: legionella can typically multiply where water is held at temperatures roughly between 20°C and 45°C. Water that is not kept hot enough or cold enough sits in this growth range.
Aerosols: the system needs to be capable of creating a breathable spray or mist, such as from a showerhead, cooling tower or spa pool.
Stored or recirculated water: water that is stored, recirculated or allowed to stagnate gives bacteria time and conditions to grow.
Nutrients and biofilm: sludge, scale, rust, organic matter and biofilm provide nutrients that support bacterial growth.

Risk is often highest where these factors overlap, for example in a rarely used shower fed by a poorly maintained storage tank, or in a section of dead-leg pipework where water sits undisturbed at a warm temperature. Periods of low or no occupancy, such as a building closure, can also allow stagnation and increase risk unless outlets are managed.

How Legionella Is Controlled In Buildings

The practical aim of legionella control is to stop the bacteria multiplying to harmful levels and to limit the creation of aerosols where possible. Control is about active management of the water system, not simply holding documents.

Recognised control measures set out in HSE guidance include:

keeping hot water hot and cold water cold, so temperatures stay outside the range where legionella readily multiplies, so far as is reasonably practicable
keeping water moving and avoiding stagnation, including flushing outlets that are used infrequently
removing redundant pipework and dead legs where water can sit undisturbed
cleaning and descaling showerheads and taps
maintaining the condition of storage tanks, including lids and screens to keep out contamination
maintaining an up-to-date schematic or asset register showing the system layout
monitoring the system, for example checking temperatures and tank condition

These measures are usually brought together in a written scheme of control, which sets out what will be done, by whom, how often, and how it will be checked. Monitoring only adds value if the results are actually reviewed. A recorded contractor visit or a set of temperature readings does little to control risk if failed or out-of-range results are never acted on.

The right combination and frequency of controls should flow from the risk assessment and the type of system, rather than a single fixed template applied to every building.

When Legionella Testing Is And Is Not Required

A common misunderstanding is that legionella testing is a legal requirement for every water system. It is not. The core legal duty is to assess and control the risk. Routine microbiological sampling for legionella is not automatically required and is only appropriate in certain circumstances.

In a well-managed hot and cold water system, routine sampling is often unnecessary, because control is demonstrated through temperature monitoring and the other measures in the written scheme. Sampling is more likely to be appropriate where:

control measures cannot rely mainly on temperature, for example where water is stored or distributed at temperatures that could support growth
the system is more complex or higher risk, such as some cooling systems or spa pools
there is reason to doubt that control is being achieved
there are susceptible people, such as in some healthcare settings, where additional assurance is appropriate

Where sampling is carried out, it should use appropriate methods, and a single sample should not be treated as proof that a system is safe. One result reflects a point in time and does not demonstrate ongoing control. The value of any sampling lies in how adverse results are investigated and acted on.

There is no universal legal frequency for testing. How often any monitoring, checking or sampling should happen depends on the risk assessment, the type of system and the control regime, rather than a fixed interval that applies everywhere.

Records And Evidence Dutyholders Should Keep

Records help a dutyholder show that risk is being managed, but they do not prove compliance on their own. They only carry weight if the underlying control measures are actually being delivered and reviewed. Relevant records and evidence typically include:

the legionella risk assessment, with clear review dates
the written scheme of control
a system schematic or asset register showing outlets, tanks, calorifiers and any dead legs
the name of the responsible person and evidence of their competence and training
monitoring records, such as temperature checks, flushing of infrequently used outlets, tank inspections and showerhead cleaning and descaling
any microbiological sampling results and the response to adverse results
maintenance and remedial records, ideally with an action tracker showing what has been completed
cooling tower notification records where applicable

The assessment and controls should be reviewed when circumstances change. Triggers for review can include alterations to the system, a change of use, occupancy changes, refurbishment, periods of low or no use, failed monitoring results, a suspected case, or new guidance.

Common weaknesses show up in practice: a risk assessment that is still in date but no longer reflects a building after alterations, actions that were identified but never closed out, or contractor temperature readings that were recorded but never reviewed when they fell out of range. Records that look complete on paper can still fail to reflect real control.

What Happens When A Case Is Linked To A Workplace Or Premises

Legionnaires’ disease is a notifiable disease in England, which means clinicians and laboratories have duties to report cases to public health authorities. A report can trigger a public health investigation, particularly where cases appear to be linked or a common source is suspected.

Where a case is suspected of being connected to a particular building or activity, several things may follow:

UKHSA or the relevant public health team may investigate the case and look for a possible source, especially if it is part of a cluster or outbreak.
Under RIDDOR, reporting may be required where the illness can reasonably be attributed to a person’s work with a water system. This does not apply to every case, only where the specific work-related criteria are met.
The HSE or local authority may become involved as the enforcing authority, depending on the type of premises.

For a dutyholder, the practical priorities are to protect people, cooperate fully with UKHSA and the relevant enforcing authority, and preserve records and evidence about the water system. This includes the risk assessment, the written scheme of control, monitoring records and any sampling results. Where a case is linked to premises, accurate and up-to-date records that reflect what was actually being done become particularly important.

Notification arrangements and the relevant public health body vary between the UK nations, so the specific process should be checked according to where the premises are located.

Common Misunderstandings About Legionnaires’ Disease

Several persistent myths surround Legionnaires’ disease and how it is managed. Clearing these up helps both individuals and dutyholders act sensibly.

You catch it from drinking contaminated water. Infection comes from inhaling contaminated aerosols, not from drinking water.
It spreads from person to person like flu. It is not usually contagious between people, although rare exceptions have been reported.
Symptoms confirm the illness. Symptoms overlap with flu, COVID-19 and other chest infections, so diagnosis requires clinical assessment and specific tests.
A risk assessment on its own means the premises is compliant. The assessment must lead to proportionate control, monitoring and review to be meaningful.
Legionella testing is legally required for every system. Sampling is only needed in specific circumstances; the core duty is to assess and control the risk.
A single water sample proves the system is safe. One result reflects a single point in time and does not demonstrate ongoing control.
The landlord is always responsible. Responsibility depends on who controls the system and the terms of any lease.
Appointing a contractor discharges the duty. The dutyholder keeps responsibility for managing and reviewing control.

Frequently Asked Questions

What are the first signs of Legionnaires’ disease?

Early signs are usually flu-like and may include a high temperature, headache, muscle aches and tiredness. These often develop before respiratory symptoms such as a cough or breathlessness. Because they are non-specific, they do not confirm the illness.

How quickly do Legionnaires’ disease symptoms appear?

Symptoms usually appear between 2 and 10 days after exposure, though the period can occasionally be longer. A longer gap between a possible exposure and becoming unwell does not rule it out.

Can Legionnaires’ disease feel like flu?

Yes. The early stage often feels like flu, with fever, aches and tiredness. This is one reason it can be difficult to recognise before more serious respiratory symptoms develop.

Can Legionnaires’ disease cause confusion?

It can. Confusion or changes in mental state can occur, particularly as the infection becomes more severe. Sudden confusion alongside a chest infection should prompt urgent medical attention.

Can Legionnaires’ disease cause diarrhoea?

Yes, digestive symptoms such as diarrhoea, nausea or vomiting can occur in some cases, sometimes alongside respiratory symptoms.

Is Legionnaires’ disease contagious?

It is not usually spread from person to person. People generally catch it by inhaling contaminated water droplets rather than through contact with someone who is unwell, although rare person-to-person cases have been reported.

Can you catch Legionnaires’ disease from drinking water?

No. It is caught by breathing in contaminated aerosols, not by drinking water. Swallowing water is not the usual route of infection.

Can you catch Legionnaires’ disease from a shower?

It is possible where a shower produces a spray of water in which legionella has been allowed to multiply, particularly if the shower is used infrequently and the system is poorly maintained. This is why flushing and cleaning of outlets forms part of legionella control.

Can you catch Legionnaires’ disease from a hot tub or spa pool?

Yes. Spa pools and hot tubs can create aerosols and provide warm water conditions that support bacterial growth if not properly managed, which is why they are treated as a higher-risk system.

Who is most at risk from Legionnaires’ disease?

People over 45, smokers and heavy drinkers, and those with conditions such as chronic lung disease, kidney disease, diabetes or a weakened immune system are more likely to become seriously ill. These factors describe susceptibility rather than certainty for any individual.

When should I contact NHS 111 or a GP?

Contact your GP or NHS 111 if you have symptoms of a chest infection that are getting worse, such as a high temperature with a cough, breathlessness or chest pain. Seek urgent help or call 999 for severe breathing difficulty, sudden confusion or if someone becomes seriously unwell.

What should I tell my doctor if I think I was exposed?

Mention any recent stays in hotels or holiday accommodation, use of a hot tub or spa pool, hospital or care home stays, or visits to buildings with showers, cooling towers or other water systems. This information can help a clinician decide whether testing is appropriate.

What should an employer do if Legionnaires’ disease is suspected?

The priorities are to protect people, cooperate with UKHSA and the relevant enforcing authority, and preserve records and evidence about the water system, including the risk assessment, written scheme of control and monitoring results. Reporting duties may also apply where the case is linked to work.

Is Legionnaires’ disease reportable under RIDDOR?

It may be. Reporting under RIDDOR applies where a case of legionellosis can reasonably be attributed to a person’s work with a water system. It does not apply to every case, only where the specific work-related criteria are met.

Is Legionnaires’ disease a notifiable disease in England?

Yes. In England it is a notifiable disease, so clinicians and laboratories have duties to report cases, which can trigger public health investigation. Notification arrangements differ between the UK nations.

What temperature controls legionella?

Legionella can typically multiply where water sits roughly between 20°C and 45°C. Control generally involves keeping hot water hot and cold water cold, so far as is reasonably practicable, so that temperatures stay outside that growth range.

Is legionella testing a legal requirement?

Routine legionella sampling is not automatically a legal requirement. The core legal duty is to assess and control the risk. Sampling is only appropriate in certain circumstances, depending on the system and control regime.

How often should legionella testing be carried out?

There is no universal fixed frequency. Where sampling is appropriate, how often it is done should be driven by the risk assessment, the type of system and the control measures in place, rather than a set interval that applies to every building.

What is ACOP L8?

ACOP L8 is the Approved Code of Practice on controlling legionella bacteria in water systems. It explains how to comply with the law and has a special legal status: following it is normally enough to comply, and a court may treat a failure to follow it as evidence of a breach unless compliance was achieved another way.

What is HSG274?

HSG274 is HSE technical guidance that supports L8. It is published in three parts covering evaporative cooling systems, hot and cold water systems, and other risk systems such as spa pools and humidifiers. It is guidance rather than law in itself.

Do landlords need a legionella risk assessment?

Where a landlord has a duty in relation to a let property, they are expected to assess and manage the risk from water systems. Whether the duty sits with the landlord or the tenant depends on who controls the system and the terms of the tenancy, so it should be checked case by case.

Do care homes need a legionella risk assessment?

Yes. Care homes have susceptible residents and are expected to assess and manage water safety. CQC-registered providers are also expected to provide safe premises, which includes water safety, alongside the general legal duties.

What records should dutyholders keep?

Useful records include the risk assessment and its review dates, the written scheme of control, a system schematic, the name and competence of the responsible person, monitoring records, any sampling results and the response to them, maintenance and remedial records with an action tracker, and cooling tower notifications where relevant.

When To Seek Competent Support

If you are unwell with symptoms that could suggest a chest infection, particularly if they are getting worse, seek medical advice from your GP or NHS 111, and urgent help for severe symptoms. Symptoms alone cannot confirm Legionnaires’ disease, and only a clinician can diagnose it.

If you are an employer, landlord, managing agent or other person in control of premises, competent support may be sensible where you are unsure whether your water systems create a foreseeable risk, whether your existing risk assessment still reflects your building, or whether your control measures and records genuinely demonstrate that the risk is being managed. A competent adviser can help you assess and control the risk, but the legal responsibility stays with you as the dutyholder.