This is an acute viral infection of the larynx, trachea and bronchi (acute viral laryngotracheobronchitis). With croup the area around the vocal cords is swollen as is the area just below the cords. Viral croup typically presents in children around 2 years of age (between 6 months and 6 years), especially in autumn. Viral croup is usually mild and the signs of croup usually clear in a few days but may recur. Some children develop viral croup whenever they have a common cold or pharyngitis.

The most common cause of viral croup is an infection with parainfluenza virus.