Croup, or laryngotracheobronchitis, is an infection of the trachea (the upper part of the windpipe, just below the vocal cords) which often also involves the vocal cords themselves as well as the large air passages in the lung.
Croup is usually caused by a viral infection. Back when measles was common, the measles virus often caused croup. Now that we immunize against measles, we hardly ever see it in the United States -- when we do see it, it is in children who did not receive the vaccine (another reason to immunize). The most common cause of croup nowadays are the parainfluenza viruses, which despite their name are not related to influenza (in fact, the two families of viruses are very different in structure and function). There are several different types of parainfluenza viruses. Unfortunately we do not yet have vaccines for any of them. The influenza viruses can also cause croup, as can the adenoviruses
When you get croup, you usually start out with cold symptoms: cough, congestion, and a runny nose with clear discharge. As the trachea's walls swell, the inside of the trachea narrows. Unlike asthma, where the small air passages in the lung become inflamed and narrow, croup does not make it hard to breathe out. (The wheezing of asthma is caused by air being forced out through the narrowed small airways.) In croup, the narrow trachea makes it hard to inhale, and you can hear a wheezing-like sound, or stridor, when you try to inhale. The swelling of the vocal cords makes you hoarse, and can make you sound like a seal barking. With bigger people (adolescents and adults) the trachea is large enough that breathing isn't affected as much as in small children, but the inflammation of the vocal cords results in laryngitis (you can't talk).
Since it's caused by a virus, there is no medicine that can stop croup completely. The first thing I usually suggest when a patient of mine has a croupy cough is to turn on the shower in your bathroom as hot as it will go, steam up the bathroom thoroughly, and then take your child into the steamy bathroom (not under the hot water!) and let him/her breath the steam. Another tactic is to breathe the mist from a cool-mist humidifier; the moisture helps soothe the inflamed mucous membranes in the airways. A third method is to let your child breathe cold air, such as outside air in winter. (Often parents will take their child to the emergency room for bad croup, and the cold air from the trip outside is enough to relieve the swelling.) If those methods do not work, there are medicines which will reduce the swelling. (This is similar to the treatment for asthma, but the medicines used for asthma do not necessarily work on croup. NEVER use a medicine for something other than what your doctor prescribed it for without asking your doctor first!)
Occasionally croup can make it so hard to breathe that a child may not be getting enough air; this is one reason why we sometimes have to put "croupers" in the hospital. Often we put children who are admitted for croup in a "cool-mist tent" (with oxygen if needed), and give the medicines I mentioned above. Usually, though, the steamy bathroom and the cool-mist humidifier work pretty well. I usually tell the parents of my patients to call me if their children are still having a lot of coughing after the steamy bathroom.