In a study led by Forest W. Arnold of the University of Louisville, atypical treatment was defined as the use of any antibiotic regimen that contained a macrolide, fluoroquinolone or tetracycline -- all broad-spectrum antibiotics. Atypical pneumonias are those diseases caused by organisms other than the so-called typical bacteria, viruses or fungi.

In a pool of 4,337 patients, Arnold and associates found a 22 percent global incidence of atypical pneumonia infections.

Hospitalized patients treated with antimicrobials against atypical pathogens reduced the time to clinical stability from 3.7 days to 3.2; their hospital stay from 7.1 days to 6.1; total mortality from 11.1 percent to 7 percent; and CAP-related mortality from 6.4 percent to 3.8 percent.

The study appears in the American Journal of Respiratory and Critical Care Medicine, published by the American Thoracic Society.