Croup is a viral infection of the voice box (larynx) and windpipe (trachea) associated with signs of a respiratory infection, such as a runny nose or cough. It's also a very common ailment of childhood. Usually the first indication of croup is a cough that sounds like the bark of a dog or seal.Your child may have trouble breathing because the tissue around the larynx is inflamed, constricting the windpipe, and because the bronchial passages are blocked with mucus.roup lasts for five or six days and is highly contagious to other children. It usually affects children between the ages of 3 months and 6 years (the average age is 2 years), whose small windpipes and bronchial passages are most vulnerable to blockage. Most cases are mild and can be managed at home.