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The recent availability of diagnostic polymerase chain reaction-based assays for the detection of viral pathogens has allowed us to identify the infectious agents present in a large percentage of culture-negative specimens. The information gained by these studies has led to a reassessment of the prevalence of individual viruses in lower respiratory tract infections in both normal and immunocompromised children. Pathologic examination of less severe cases identified by more sensitive methodologies in immunocompetent children is now possible and has prompted the realization that many of the classic features of particular viral infections in the lung are in fact hallmarks of fulminant disease. In this article we review the body of literature discussing these anatomic findings, the approaches taken by those investigators, and the types of reagents that are commercially available.