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. In adults and older children, reinfections are frequently asymptomatic but may present as common colds or, less often, pharyngitis, tracheobronchitis, or influenzalike illness. Parainfluenza viruses cause serious lower respiratory tract disease, including exacerbations of chronic airway disease and pneumonia in adults. Severe pneumonia, sometimes in the absence of upper respiratory tract illness, occurs in immunosuppressed adults and children, particularly after bone marrow transplantation.