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Prospective data on viral etiology and clinical characteristics of bronchiolitis and upper respiratory illness (URI) in infants are limited.This prospective cohort enrolled previously healthy term infants during inpatient or outpatient visits for acute URI or bronchiolitis during September to May 2004 to 2008. Illness severity was determined using an ordinal bronchiolitis severity score. Common respiratory viruses were identified by real-time reverse-transcriptase polymerase chain reaction.Of 648 infants, 67% were enrolled during inpatient visits and 33% during outpatient visits. Seventy percent had bronchiolitis, 3% croup and 27% URI. Among infants with bronchiolitis, 76% had respiratory syncytial virus (RSV), 18% human rhinovirus (HRV), 10% influenza, 2% coronavirus, 3% human metapneumovirus and 1% parainfluenza virus. Among infants with croup, 39% had HRV, 28% parainfluenza virus, 28% RSV, 11% influenza, 6% coronavirus and none human metapneumovirus. Among infants with URI, 46% had HRV, 14% RSV, 12% influenza, 7% coronavirus, 6% parainfluenza virus and 4% human metapneumovirus. Individual viruses exhibited distinct seasonal, demographic and clinical expression.The most common infections among infants seeking care in unscheduled medical visits for URI or bronchiolitis were RSV and HRV. Demographic differences were observed between patients with different viruses, suggesting that host and viral factors play a role in phenotypic expression of viral illness.