What is it?
Croup is a viral infection that causes swelling of the windpipe just below the vocal cords. Several types of viruses may cause this infection, but the most common cause is a virus called parainfluenza. The viruses that cause croup are the same ones that cause colds in older children and adults. Although family members who catch the same cold virus often have similar symptoms, this may not always be the case because it is the nature of cold viruses to cause different symptoms in different people. Thus, while a particular virus may cause achiness and fever in you, it may give me just a runny nose and sore throat, give one child "pink eye" and another child, croup. The point is that croup is a description of symptoms rather than a name for a disease caused by a specific virus.
What are the symptoms?
Croup usually appears after your child has had a cold (fever, runny nose, etc.) for several days. As the swelling of the windpipe increases, a high-pitched cough develops that sounds much like a seal's bark. For most children, this is the extent of problems that occur, and the "barkiness" of the cough lasts for a couple of days, although the cough itself may last for a few days longer. However, for some children, the swelling of the windpipe becomes severe enough to impair the flow of air. This may cause noisy breathing upon inspiration called stridor and a "pulling in" of the area beneath the Adam’s apple or between the ribs. All these signs point to an increased effort to breathe to get air past the swelling of the windpipe.