This review on bronchiolitis in young children considers the viruses involved, the current understanding of pathogenesis, host genetic factors and the environment, and the role of season, race, and sex on attack rates and subsequent episodes of wheezing. Few diseases have a greater effect on the health of young children than viral lower respiratory tract illness. Approximately 800,000 children in the United States, or approximately 20% of the annual birth cohort, require outpatient medical attention during the first year of life because of illness caused by respiratory syncytial virus (RSV). 1 Between 2% and 3% of all children younger than 12 months of age are hospitalized with a diagnosis of bronchiolitis, which accounts for between 57,000 and 172,000 hospitalizations annually. 1 – 4 Estimated nationwide hospital charges for care related to bronchiolitis in children younger than 2 years of age exceeded . . .