An acute respiratory infection caused by the bacteria Legionella pneumophilia, which causes a serious pneumonia . The bacteria has been found in water delivery systems and can survive in the warm, moist, air conditioning systems of large buildings including hospitals. The infection is transmitted through the respiratory route. Person to person spread has not been proven. From the onset of symptoms, a worsening of the condition is typical during the first 4 to 6 days, with improvement starting in another 4 to 5 days.
Most infection occurs in middle-aged or older people, although it has been reported in children.
Typically, the disease is LESS severe in children. Active surveillance of infections that were acquired within a hospital can lead to the treatment of contaminated water delivery systems. Detection and treatment of sources outside hospitals usually occurs during or after an epidemic has happened. Chest X-ray shows pneumonia . Arterial blood gas analysis may show low concentrations of oxygen.