Term, non-low birth weight, otherwise healthy infants were enrolled along with their biological mothers, at a single academic institution, Vanderbilt Children's Hospital, at the time of an acute visit (hospitalization, emergency department or unscheduled outpatient visit) for presumed viral bronchiolitis or upper respiratory tract infection (URI) during respiratory viral seasons September through May 2004-2008. Inclusion and exclusion criteria are outlined in Table 1 . Because of the grant funding start date, the first study season did not begin until November 2004. Recruitment was solely hospital-and clinic-based, and was performed 7 days/week during the first 2 years of cohort accrual, and 5 days per week for the two subsequent years. Screening and recruitment were done by experienced Congenital or acquired chronic heart or lung disease, prior requirement for mechanical ventilation for cardiac or pulmonary disease, immunodeficiency, neurologic disease with possible aspiration, significant gastroesophageal reflux disease felt to contribute to pulmonary disease, tracheomalacia Ever received one or more doses of RSV-IVIG or palivizumab Prior study inclusion Fever and neutropenia Children whose parents or guardians were not able to understand the consent process, or a language barrier † research nurses using computerized medical charts to screen infants with presumed respiratory viral illness.