 Children younger than one year of age with influenza will be recruited for this study. Participants will receive the usual care for influenza with the addition of oseltamivir. Oseltamivir will be given orally for 5 days to children with human influenza, 7 days to children with H1N1 influenza in Vietnam, and 10 days to children with avian influenza. Participants will need to remain in the hospital between 5 and 12 days, depending on their illnesses. 