Respiratory syncytial virus (RSV), originally recovered from a colony of chimpanzees with coryza and designated chimpanzee coryza agent, and human parainfluenza virus types 1, 2, 3, and 4 have been known primarily as respiratory pathogens in young children. They are now recognized as important pathogens in adults as well. Adults infected with these viruses tend to have more variable and less distinctive clinical findings than children, and the viral cause of the infection is often unsuspected. The consistency of the annual outbreaks of these agents and the frequency of reinfection suggest that they impose a considerable, but ill-defined, disease burden throughout life.

RSV, followed by the parainfluenza viruses, is the chief cause of hospitalization for respiratory tract illness in young children. In the 1980s an estimated 100,000 children were hospitalized with RSV infection in the United States annually, at a cost of $300 million. In 1991 it was estimated that infection of children with parainfluenza virus types 1 and 2 accounted for 250,000 visits to emergency rooms, 70,000 hospitalizations, and $190 million annually. RSV and parainfluenza viruses are also leading causes of hospitalization in adults with community-acquired respiratory disease.Despite four decades of efforts, there are no effective means to control RSV and parainfluenza virus infections. The development of vaccines has been confounded by the lack of durable immunity, even after natural infection, and the diversity and ubiquity of populations at risk for infection.

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