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Very young children, generally under 6-9 months old, are sometimes prone to a viral infection known as bronchiolitis. This infection is most often caused by respiratory syncitial virus (RSV), a common cold virus which is responsible for over half of all routine viral upper respiratory infections (that is, colds). Other causes (in descending order of frequency) include parainfluenza virus (causative agent of most croup), rhinovirus (the common cold virus), adenovirus (colds, flu-like illness, pinkeye), influenza virus, and mycoplasma pneumoniae.
The infant with bronchiolitis usually comes to the doctor with the story that she was well until a few days earlier, when she got a little fever and a clear runny nose. Mother thought it was an ordinary cold until the child began to have more cough, which sounded chesty and wheezy. Now she notes that the child seems to be breathing hard and is sleeping restlessly, and has trouble nursing because the baby's so short of breath.
This virus attacks the smallest breathing tubes, the bronchioles (bronk-ee-oles), which lead directly to the tiny air sacs. This causes swelling, mucus accumulation, and obstruction of airflow, especially airflow out of the air sacs. This produces a characteristic wheezing sound in the chest, which is audible in the breathing and in the cough. There is of course also involvement of the bronchi (bronk-eye), the larger breathing tubes, as well - so we can say that there is also a bronchitis. Thus the concomitant rattly cough of bronchitis, with coarse mucus in the bronchial tubes (bronchi), as well as the wheezing.
This infection is a leading cause of hospitalizations in young children in the winter. Ordinarily, it is a fairly benign illness, but if the infant is very young, or has had lung problems (usually premature nursery graduates), the child may get into serious trouble with breathing.
Unfortunately, as with all viral infections, bronchiolitis has to "run its course." There's no really effective home treatment or medication, and although bronchodilators such as albuterol are sometimes tried, they have been shown to be of little benefit. Steroids have been tried as well, both systemic (oral or intravenous) and nebulized, but scientific studies (the ones that count) have shown no real benefit from them and the American Academy of Pediatrics guidelines discourage their use.
How bronchiolitis relates to a later risk of asthmatic wheezing is still open to debate. Children with a history of bronchiolitis in infancy are more likely to wheeze later in childhood. Some but not all of these children will go on to have asthma after an early attack of bronchiolitis, but probably only if there is a strong family history of allergic disease.
See also bronchitis.