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Legionella is the name given to the group of bacteria which give rise a number of illnesses, including Legionnaires' disease – a potentially fatal pneumonia type of infection of the lower respiratory tract. Legionellosis is the term given to the group of diseases caused by the bacteria and include other less serious diseases such as Pontiac Fever and Lochgoilhead fever.

Legionella pneumophila

Legionella is common in many environments, with at least 50 species and 70 serogroups identified.

Most cases of Legionnaires' disease, are caused by Legionella pneumophila, but there are many other species of the organism and mild illnesses, other than pneumonia, may be caused by these organisms.

Legionella bacteria occur naturally and are widespread in the natural environment. They are present in all watercourses such as rivers, lakes and reservoirs and can even be washed in through the mains supply.

Such bacteria have most probably been around for thousands, if not millions of years, and have only started to cause problems in the last 70 years or so due to the way water is used and stored in the modern environment. Because Legionella pneumophila is found in natural water supplies and soil and it is also found in many recirculating water supply systems, which has prompted guidance on the precautions required to minimise the risk of Legionella infection

Legionella pneumophila

One of the key things to remember in assessing risks and managing Legionella is that the bacteria must physically enter the lungs in order for someone to contract Legionniare’s disease. Normally for this to happen, the water must be in a fine mist or spray, usually referred to as an ‘aerosol’.

To put this into context, imagine having a shower – it is not the droplets that hit you on the head we are interested in but the fine mist that forms in front of your eyes. If this is infected with Legionella then inhalation of this mist could result in Legionnaires' disease

There is no evidence that the disease is contagious via person to person but there have been reported cases of Legionnaires' disease being contracted through drinking contaminated water by a process called aspiration.

Further Information

Legionnaires' disease

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