 This was a study involving the emergency department and outpatient clinics of the David Grant United States Air Force USAF Medical Center, a tertiary care facility. Patients 18 years of age or older with acute respiratory tract symptoms and positive chest radiographs from October 1, 2004 through May 31, 2005 were included as positive cases. Controls were randomly selected from a review of negative chest radiograph reports with a clinical history of an acute respiratory illness over the same time period. Equivocal radiographs were treated as positive cases. Patients were excluded if they were under the age of 18 or in suspected cases of hospital-acquired or aspiration pneumonia. 