One in three children can be anticipated to develop a lower respiratory tract infection (LRI) in the first year of life. Denny and Clyde reported 6000 cases of LRI in an outpatient practice in North Carolina. Of this group, 15% had croup, 34% had tracheobronchitis, 29% had bronchiolitis, and 29% had pneumonia. The overall attack rate for pneumonia in all ages was four to six cases per hundred children per year.

An LRI is defined as any respiratory tract infection involving the larynx or structures distal to it. Four clinical syndromes have been described: croup, tracheobronchitis, bronchiolitis, and pneumonia. Croup is characterized by hoarseness, cough with inspiratory stridor, and laryngeal obstruction. Tracheobronchitis leads to cough with rhonchi, but without laryngeal obstruction. Croup and acute tracheobronchitis primarily are the result of extension of viral upper respiratory infections. They generally are self-limited and require no more than symptomatic treatment. Neither will be discussed further.

Bronchiolitis is an acute infectious inflammation of the lower airways. The classic syndrome is defined as the first episode of wheezing in a child younger than 2 years of age. Viral infection of the airways, most often respiratory syncytial virus (RSV), is the most common etiology of bronchiolitis. However, other viral agents and chronic inflammatory diseases, including cystic fibrosis, ciliary dyskinesia, and recurrent aspiration, also can be associated with bronchiolitis.

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