Legionnaires’ disease is the common name given to a pneumonia like bacterial infection caused by the pathogenic bacteria Legionella pneumophila.

It takes its name from the case where it was first identified as a causative agent of infection following an outbreak at a convention of a military veterans association, the American Legion, in 1976.

Legionella pneumophila is only one member of a genus of bacteria that can cause disease and illness. To date over 50 species in this genus have been identified with over 24 associated with human infections.

Legionnaires’ disease, along with other conditions such as Pontiac fever and Lochgoilhead fever are collectively known as Legionellosis. Whilst Pontiac and Lochgoilhead fevers produce mild respiratory illnesses Legionnaires’ disease causes acute pneumonia resulting in a mortality of between 5% and 50%.

Healthy individuals can usually fight off infection from the bacteria. However, older people, especially males, and those with underlying health issues or a suppressed system are particularly at risk.

The incidence of infection of those exposed (morbidity rate) to Legionella pneumophila is typically 5%, although this is greatly dependent upon the susceptibility of the exposed population. Although they are much less severe diseases the morbidity rates for Pontiac and Lochgoilhead Fevers are much higher at 70 to 90% of those exposed to the bacteria going on to develop the disease.

The bacteria responsible for these infections naturally occur at low levels in mud, soil and water courses and at these levels they pose very little threat of human infection. However, the conditions found in man-made environments such as cooling towers, spa pools and hot and cold water systems can favour a rapid rise in the numbers of bacteria to levels where dissemination and infection becomes possible.