Viral infections account for most lower respiratory tract infections in young infants (less than 60 days of age). Respiratory syncytial and parainfluenza 3 virus infections accounted for approximately three fourths of the identified virus infections. Other agents were isolated infrequently.
Overall, viruses were recovered from 21% of the infants with pneumonia and 25% of the infants with bronchiolitis. Sixteen (18.0%) of 89 infants from whom paired sera were collected had diagnostic antibody rises. Four of five infants with bacteremia also had virus infections. Viral infections of the respiratory tract may play a role in the dissemination of bacterial disease.