Parainfluenza is transmitted by direct contact or by inhalation of contaminated airborne droplets. Paramyxoviruses occur in five serotypes—1, 2, 3, 4a, and 4b—that are linked to several diseases: croup (types 1, 2, and 3), bronchiolitis (types 1 and 3 particularly, but all may cause it), tracheobronchitis (type 3); pharyngitis (types 1, 2, 3, and 4a and 4b), bronchitis (types 1 and 3), and pneumonia (types 1 and 3). Parainfluenza type 3 ranks second to respiratory syncytial viruses as the most common cause of lower respiratory tract infections in children. Types 4a and 4b rarely cause symptomatic infections in humans.

Parainfluenza is rare among adults but widespread among children, especially boys. By age 8, most children demonstrate antibodies to parainfluenza types 1 and 3. Most adults have antibodies to all five types as a result of childhood infections and subsequent multiple exposures. Incidence rises in the winter and spring.

Parainfluenza infection in the respiratory tract leads to the secretion of high levels of inflammatory cytokines, with a peak duration of secretion 7 to 10 days after initial exposure. Increasing levels of certain chemokines are detected in the nasal secretions of pediatric patients. These chemokines are responsible for pathologic changes in the respiratory tract and the clinical manifestations of this condition. The features include airway inflammation, necrosis and sloughing of respiratory epithelium, edema, excessive mucus production, and interstitial infiltration of lung. Edema of the mucus layer causes swelling (vocal cords, larynx, trachea, and bronchi), which can lead to airflow obstruction and subsequent stridor.

Complications of parainfluenza include croup, bronchiolitis, and pneumonia.

After a short incubation period (usually 3 to 6 days), signs and symptoms emerge that are similar to those of other respiratory diseases: sudden fever, nasal discharge, reddened throat (with little or no exudate), chills, and muscle pain. Bacterial complications are uncommon, but in infants and very young children parainfluenza may lead to croup or laryngotracheobronchitis. Re-infection is usually less severe and affects only the upper respiratory tract.