Caused by the legionella bacterium strain. Patients affected by Legionella pneumonia inhale the bacteria, which is why this severe type of pneumonia is common among older adults, smokers and immunocompromised people.
The bacterium has been associated with cooling towers and evaporative condensers that form part of the air conditioning system. The cooling towers circulate large quantities of warm water where bacteria multiply easily. The escaping water carries the bacteria in the air. The disease can be contracted if people inhale high concentrations of bacteria. Hot tubs and spas are also suspected as possible sources of infection.
Severe Legionnaires' disease has an overall mortality rate of 10% to 30%, and 30% to 50% of patients require admission to an intensive care unit. Almost all reported outbreaks of Legionella in the USA and Europe have occurred during the summer and autumn. One of the most important determinants of outcome is the early initiation of adequate therapy after admission. The early symptoms of Legionella pneumonia include malaise, myalgia, anorexia, headache, profuse sweating and chills. In some series patients have developed confusion or delirium during the first week of illness, and diarrhoea, abdominal pain, and vomiting may also result.