Croup can be caused by a variety of different viruses. The most common is the parainfluenza virus, but others may include:
Your child may become infected through direct contact with a person, or through the secretions of another person who is infected with the disease. The infection begins in the upper respiratory tract and then slowly spreads down the tract (unlike a cold, which stays in the upper respiratory tract). Swelling affects the area around your child’s larynx (voice box) and into the trachea (windpipe).
Younger children are more affected by croup because their airways are smaller. Therefore, a small amount of swelling can cause a large amount of obstruction in their airways.