Croup usually results from a viral infection but can also be caused by bacteria, allergies, and inhaled irritants. Parainfluenza viruses are the cause in 75% of croup cases; adenoviruses, respiratory syncytial virus (RSV), influenza, and measles viruses account for the rest.

Croup is a childhood disease affecting boys more than girls (typically between ages 3 months and 5 years) and usually occurs during the winter. Up to 15% of patients have a strong family history of croup.

Children may develop complications, such as respiratory distress, respiratory arrest, epiglottiditis, bacterial tracheitis, atelectasis, and dehydration.

The onset of croup usually follows an upper respiratory tract infection. Clinical features include inspiratory stridor, hoarse or muffled vocal sounds, varying degrees of laryngeal obstruction and respiratory distress, and a characteristic sharp, barking, seal-like cough. These symptoms may last only a few hours or can persist for a day or two. As it progresses, croup causes inflammatory edema and, possibly, spasm, which can obstruct the upper airway and severely compromise ventilation. (See How croup affects the upper airway.)