A parainfluenza virus is a cause of viral croup, which commonly affects young children 6 months to 3 years of age mainly in the fall and winter months. More boys develop croup than do girls. Other viruses that may cause viral croup include respiratory syncytial virus (RSV), rubeola virus, adenovirus, and other influenza viruses.

The upper airway signs of croup are caused by edema of the subglottic space. Before the barking cough and stridor (high-pitched harsh noise) characteristic of croup, the child may have signs of an upper respiratory tract infection. Stridor increases when the child is agitated and crying. As the edema increases, stridor may occur when the child is quiet and resting. The parent/caregiver will also notice retractions, air hunger, and cyanosis. This child needs emergent care. A way to distinguish viral croup from epiglottitis is to assess for a cough and drooling. With croup the child will have a cough but no drooling. Usually, with epiglottitis the child will not cough but will drool.