Legionnaires’ Disease is a potentially fatal form of pneumonia caused by the legionella bacterium. The majority of cases are reported as single (isolated) cases but outbreaks can occur. 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. In 2008 there were a total of 364 cases of Legionnaires' disease reported of which 34 (10% )were fatal.

The bacterium Legionella pneumophila and related species is found naturally in environmental water sources and usually in low numbers. In these environments they pose no known threat however if they enter man made systems such as domestic hot and cold water systems they can if conditions are favourable, colonise those systems and multiply to large numbers.If the systems contain primary aerosol generators such as showers or spray taps, contaminated respirable droplets or aerosols can be inhaled which may cause legionnaires’ disease, particularly if the person in contact is a higher risk individual.

For this reason the Health & Safety at Work Act 1974, the Control of Substances Hazardous to Health (COSHH) Regulations 2002 and the  HSC - Approved Code of Practice & Guidance document  L8 -Legionnaires disease: The control of Legionella bacteria in water systems’ all combine to give employers and those responsible for buildings a legal responsibility with appropriate advice and guidance on how to effectively manage the risk..