Infectious interstitial pneumonia is most often the result of viral infection. Several viruses produce this pattern, in addition to causing airway inflammation and necrosis. In severe cases, the infection may result in diffuse alveolar damage. The overall pattern of injury and the presence of characteristic viral inclusions often provide clues to the etiologic agent in most cases. The most common viruses that produce interstitial inflammation are influenza and parainfluenza virus, measles, respiratory syncytial virus, cytomegalovirus, herpes simplex virus, and adenovirus. Chronic Epstein-Barr virus infection also produces an interstitial pneumonia. Other less common causes include congenital listeriosis, toxoplasmosis, rubella, and congenital varicella zoster infection.