Viral infections of lung are common, and often latent and self-limited except in immunocompromised patients, who can develop extensive pulmonary infections with severe respiratory compromise. Diagnosis of viral infection in nonimmunocompromised individuals is made by serology and viral cultures. In immunocompromised patients, time is of the essence, and transbronchial biopsy may yield a quick diagnosis based on the detection of typical viral inclusions. The diagnosis can be confirmed by immunohistochemical stains for viruses. In situ hybridization and polymerase chain reaction (PCR) tests are available for common viruses for further confirmation.

Almost all viruses induce cytopathic effects in the infected cells, some so morphologically distinct as to provide a diagnosis with confidence.

The viral pulmonary infections commonly seen in general surgical pathology practice include adenovirus, cytomegalovirus, herpes simplex virus, measles virus, respiratory syncytial virus, and, less commonly, varicella-zoster, influenza, and parainfluenza virus.

Some viral inclusions are associated with intranuclear or cytoplasmic inclusions and others with both. In the early stages of infection, the viral inclusions of different members of the herpesvirus family, that is, cytomegalovirus, herpes simplex, and varicella zoster, are difficult to distinguish from each other, and initially a specific diagnosis may not be made on transbronchial biopsy (Table 9-1).