Legionnaires’ disease is caused by bacteria that belong to the family Legionellaceae.


This family now includes over 50 species and 70 serogroups. Approximately half of these species have been implicated in human disease. Legionella pneumophila has 16 serogroups and is responsible for approximately 90% of infections. Most cases are caused by L. pneumophila, serogroup 1. Legionella species are small (0.3 to 0.9 μm in width and approximately 2 μm in length)


Legionnaires' disease is an uncommon form of pneumonia caused by the legionella bacterium. All ages can be affected but the disease mainly affects people over 50 years of age, and generally men more than women. Smokers and the immunocompromised are at a higher risk.



There are two main types of illness which can develop after being infected with the legionella bacterium:


  • Legionnaires' disease. This causes a pneumonia (lung infection). It is usually a severe illness, sometimes fatal.
  • Pontiac fever / Lochgoilhead fever. This is a mild flu-like illness caused be legionella bacteria, often affecting previously healthy and young individuals.  Symptoms can include fever, headaches and muscle aches but, unlike Legionnaires' disease, Pontiac/Lochgoilead fever does not cause pneumonia.  The illness will usually clear up without treatment within two to three days.


The early symptoms of Legionnaires' disease include a 'flu-like' illness: dry cough, fever, sweats, shivers, being off food, and feeling generally unwell. Headaches, tiredness and aches and pains are common. The sputum may become yellow/green, and is sometimes bloodstained.


  • Symptoms tend to become worse quite quickly as the infection spreads through the lung tissue. You may become breathless, breathe fast, and develop a 'tight chest'. You may have pain in the side of the chest when breathing ('pleuritic pain').
  • Common 'non-lung' symptoms include confusion and diarrhoea.
  • Complications develop in some cases and can cause heart, brain, kidney, gut or blood problems.



Deaths occur in 10-15% of the general population and may be higher in some groups of patients. The incubation period can range from 2 to 19 days with an average of 6 to 7 days after exposure.


The drugs of choice to treat Legionella belong to a class of antibiotics called macrolides. The illness is treated with an antibiotic called erythromycin but other antibiotics are often used, for example: azithromycin, clarithromycin, levofloxacin, ciprofloxacin or doxycycline. If you are pregnant, erythromycin and azithromycin are safe for the baby. Sometimes a combination of antibiotics is used, if it is not yet certain whether the infection is due to Legionella or to other bacteria.


Legionnaires' disease is a serious illness. If you were previously fit and healthy, you have about a 1 in 10 chance of dying from legionnaires' disease. If you were previously unwell, (for example, if you already have a lung disorder) then you have a higher risk of dying if you get legionnaires' disease. The outlook is best if the illness is diagnosed as early as possible, and you are treated with antibiotics as soon as possible. After recovery from legionnaires' disease, you may get symptoms such as tiredness, poor concentration, cough or mild shortness of breath. These may take several months to clear up.


People become infected when they inhale legionella bacteria which have been released into the air in aerosolised form from a contaminated source. Once in the lungs the bacteria multiply and cause either pneumonia or a less serious flu-like illness (Pontiac fever). Legionnaires’ disease cannot be contracted person to person.


However, the conditions are rarely right for people to catch the disease from the natural sources. Outbreaks of the illness occur from exposure to legionella growing in purpose-built systems where the water is maintained at a temperature high enough to encourage growth, eg cooling towers, evaporative condensers, spa pools, and hot and cold water systems used in all sorts of premises (work and domestic).


The bacteria are widely distributed in the environment. They can live in all types of water including both natural sources such as rivers, streams and ponds and purpose built systems.They survive low temperatures and thrive at temperatures between 20-45°C if the conditions are right, eg if a supply of nutrients is present such as rust, sludge, scale, algae and other bacteria. They are killed by high temperatures.



There are 4 main statutes which are relevant to the control of legionella bacteria;


  • The Health and Safety at Work Act 1974 (HSWA)
  • The Control of Substances Hazardous to Health Regulations 2002 (COSHH)
  • The Management of Health and Safety at Work Regulations 1992 (MHSWR)
  • The Notification of Cooling Towers and Evaporative Condensers Regulations 1992


All of the above have been considered by the Health and Safety Commission when writing the Approved Code of Practice (ACoP) L8, along with what practices have proven efficacy (i.e. best practice). The L8 is not however legislation.


The ACoP has been approved by the Health and Safety Commission, with the consent of the Secretary of State. It gives practical advice on how to comply with the law. If you follow the advice you will be doing enough to comply with the law in respect of those specific matters on which the code gives advice. You may use alternative methods to those set out in the code in order to comply with the law.

However, the code has a special legal status. If you are prosecuted for breach of health and safety law, and it is proved that you did not follow the relevant provisions of the code, you will need to show that you have complied with the law in some other way or a court will find you at fault.


You have to consider the risks from legionella that may affect your staff or members of the public and take suitable precautions. As an employer or a person in control of the premises (eg a landlord), you must:


  • identify and assess sources of risk;
  • prepare a scheme (or course of action) for preventing or controlling the risk;
  • implement and manage the scheme – appointing a person to be managerially responsible, sometimes referred to as the ‘responsible person’;
  • keep records and check that what has been done is effective; and
  • if appropriate, notify the local authority that you have a cooling tower(s) on site.


Assessing the risk


The risk assessment is your responsibility as the employer or person in control of the premises. You may be able to carry out the assessment yourself but, if not, you should call on help and advice from within your own organisation or, if this is not available, from specialist, accredited external sources.

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