This disease can occur sporadically or in epidemics as the first known one did, in 1976, at an American Legionnaires' convention in Philadelphia. The sporadic cases may be caused by exposure to an air-conditioning system or water tap carrying the bacteria. Legionnaires' disease is very common in certain cities such as Los Angeles and Pittsburgh and in some states such as Montana. A definitive diagnosis cannot be made, however, without chest x-rays, which show patches in the lung where air sacs are filled with fluid and debris instead of air. To determine the particular infective agent, lab tests can be done on sputum, urine, and blood samples, but these tests are not completely reliable.
Most viral pneumonias are mild.
Symptoms include fever, headache, malaise, chills, and cough. Headache, nausea, and vomiting may also occur. The symptoms of bacterial pneumonia at first resemble those of a chest cold, with a dry cough and then a sputum-producing cough. When bacterial pneumonia occurs as a complication of influenza A, flu symptoms often disappear before the patient suddenly worsens with fever, cough, and shortness of breath. In the case of Legionnaires' disease, after an incubation period of 2 to 10 days, the patient develops fever, general muscular pain, chills, abdominal pain, diarrhea, nonproductive cough, sore throat, and sometimes headache and confusion.