Azithromycin to Prevent Wheezing Following Severe Respiratory Syncytial Virus (RSV) Bronchiolitis

Inclusion Criteria: 1. Age: 1-18 months. 2. Hospitalization for the first episode of RSV bronchiolitis: * Confirmed RSV infection by positive nasal swab results (viral culture and/or direct antigen detection) from the SLCH virology lab; AND * At least two of the following symptoms/signs of bronchiolitis: respiratory rate greater than 40 breaths/minute; cough; wheezing; audible rales, crackles, and/or rhonchi; paradoxical chest movements (retractions). 3. Duration of respiratory symptoms from initiation of symptoms to admission is 5 days or less. Time of admission will define by the time the child was seen in the ED for the visit that led to hospitalization. 4. Randomization can be performed within 48 hours from time of admission (defined by time of first set of vital signs obtained on the floor). 5. Willingness to provide informed consent by the child's parent or guardian \- Exclusion Criteria: 1. Prematurity (gestational age \< 36 weeks). 2. Presence or history of other significant disease (CNS, lung, cardiac, renal, GI, hepatic disease, hematologic, endocrine or immune disease). Children with atopic dermatitis will not be excluded from the study. 3. Clinically significant gastroesophageal reflux currently treated with a daily anti-reflux medication (anti- H2 or PPI). 4. The child has significant developmental delay/failure to thrive, defined as weight \< 3% for age and gender. 5. History of previous (before the current episode) wheeze or previous treatment with albuterol. 6. Treatment (past of present) with corticosteroid (systemic or inhaled) and/or montelukast. 7. Treatment with any antibiotics in the past 2 weeks. 8. Treatment with Macrolide antibiotic (Azithromycin, clarithromycin or erythromycin) with the past 4 weeks. 9. Current treatment with any daily medication (other then albuterol, vitamins or nutritional supplements). 10. Participation in another clinical trial. 11. Evidence that the family may be unreliable or nonadherent, or may move from the clinical center area before trial completion. 12. Contraindication of use of azithromycin or any other macrolide antibiotics.