The bacteria that cause Legionnaire's disease have 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 system. Person-to-person spread has not been proved. From the onset of symptoms, the condition typically worsens 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. Risk factors include cigarette smoking; underlying diseases such as renal failure, cancer, diabetes, or chronic obstructive pulmonary disease; people with suppressed immune systems from chemotherapy, steroid medications, or diseases such as cancer and leukemia; alcoholism; being middle-aged or elderly, and in people on a ventilator for extended periods.
| 
