The incidence of croup and respiratory disease caused by type 1 or 2 virus infections is highest between the ages of six months and five years, whereas type 3 virus is an important cause of illness in infants younger than six months. Type 1 virus typically causes epidemics lasting several months, peaking in October or November of alternating years. Type 2 virus activity is more sporadic, and varying patterns have been described for types 1 and 2 viruses. Type 3 virus causes infections throughout the year, with episodic outbreaks during the spring and summer. Parainfluenza viruses are transmitted from person to person either by direct contact with respiratory secretions or by large aerosol droplets.
Transmission occurs readily in families, and reinfections in older children and adults are common.
Outbreaks occur in nurseries, day care centers, and hospitals, with attack rates of 40 percent or higher in patients with seronegative findings. The incubation period of parainfluenza viruses is approximately three to six days. Initial infections cause fever, rhinitis, pharyngitis, laryngitis, and bronchitis in children. Up to one third of infected children have evidence of lower respiratory tract involvement, typically manifested as croup in type 1 and 2 virus infections and as bronchiolitis, pneumonia, or croup in type 3 virus infections. Type 4 virus infections cause rhinitis and, infrequently, cause lower respiratory syndromes.