Croup is a common childhood ailment. Typically, it arises from a viral infection of the larynx (voice box) and is associated with mild upper respiratory symptoms such as a runny nose and cough. The key symptom is a harsh barking cough. Croup is usually not serious and most children recover within a few days. In a small percentage of cases, a child develops breathing difficulties and may need medical attention.

At one time, the term croup was primarily associated with diphtheria, a life-threatening respiratory infection. Owing to widespread vaccinations, diphtheria has become rare in the United States, and croup currently refers to a mild viral infection of the larynx.

Parainfluenza viruses are the typical cause, but influenza (flu) and cold viruses may sometimes be responsible. All are highly contagious and easily transmitted via sneezing and coughing. Children between the ages of 3 months and 6 years are usually affected, with...