Viruses account for the largest proportion of childhood pneumonia; viruses decrease in frequency as the etiology of pneumonia in healthy adults, and once again become frequent causes of death and morbidity in persons older than 60 years.

Viral pneumonia is a subset of the pneumonitides, which were at one time called atypical pneumonias. In the past, all pneumonias were labeled atypical if a bacterial pathogen could not be identified with Gram staining and if the pneumonia did not respond to antibiotics. A number of rapid tests to determine viral etiologies have now been developed. Their use in the emergency department (ED) has allowed bedside diagnosis of the etiology of viral pneumonia. This testing is important for both epidemiologic surveillance and treatment because many viral pneumonias have overlapping clinical presentations with each other and bacterial pneumonia, making diagnosis on purely clinical grounds difficult or impossible.