Several questions come to mind after reading what you wrote about RSV. What is the relationship, if any, between RSV and croup? What is croup anyway? How does one prevent it? What does one do if one's child gets it?

Picture this scene: A couple is spending a quiet evening at home with their one-year-old daughter, who has had a bit of a cold. At about 8 o'clock she begins coughing -- an unusually loud, resonant cough. Over the next few hours the sporadic cough becomes more and more frequent. She gets a fever. At 11 o'clock (when all the After Hours Clinics have closed and her doctor is in bed), her breathing becomes noisy and labored. Each breath now makes a worrisome crowing noise unlike anything the parents have ever heard. Concerned, they bundle her up and take her to the local emergency room. As they are pulling into the ER driveway, they notice that the child seems much, much better. After some heated debate (during which the little girl falls asleep peacefully), they decide to head home without being seen. An hour later, at home, the little girl is woken up by the loud cough, and her breathing is even more difficult...

These parents need to understand what's happening to their daughter. Before discussing her particular problem, it would be helpful to clarify some terms. Croup, bronchiolitis, bronchitis, and pneumonia are all words that denote the location in the body of a problem. The word croup comes from an old Germanic word for the voice box. Today, croup, or laryngotracheobronchitis, refers to swelling centered at the larynx or vocal cords. It can be caused by infection, allergy, or a foreign body. Bronchiolitis refers to inflammation in the bronchioles, the smaller airways that branch off from the main bronchi or breathing tubes. Bronchitis is inflammation in these larger, main breathing tubes. Pneumonia refers to inflammation of the lungs themselves. RSV is a specific virus that can cause infections in any of these areas.

RSV can also cause colds, sore throats, ear infections, or bronchitis, but it is usually linked with bronchiolitis, since this is the most common diagnosis in babies hospitalized with RSV. Conversely, up to 75% of cases of bronchiolitis are caused by RSV. (About 17% are caused by parainfluenza virus type 3). Kids with bronchiolitis are generally under 2 years old (it peaks at 6 months of age) and typically have sneezing and nasal discharge, followed by a wheezy cough, rapid, shallow respirations and difficulty breathing.

RSV is also responsible for about 25% of childhood pneumonias. There are a wide variety of other causes of pneumonias, each with their own typical story. As you might imagine, it can be difficult to distinguish between bronchiolitis and pneumonia. Pediatricians rely on the story, the physical exam, and sometimes X-rays to separate them.

The child in the opening scene had a classic case of croup. Croup tends to happen in children between 3 months and five years old. It is characterized by a brassy cough that sounds rather like a seal barking. It is usually (75% of the time) caused by parainfluenza viruses, but 6-8% of cases of croup are caused by RSV.