 This open-label study will assess the pharmacokineticspharmacodynamics and safety of intravenous iv Tamiflu oseltamivir in 3 cohorts of children, aged 6-12, 3-5 and 1-2 years, with influenza infection. Patients will receive iv Tamiflu therapy for 5 days 10 doses. For patients whose conditions no longer merit continued iv dosing, therapy may be switched to oral Tamiflu to complete their prescribed course of treatment. If medically necessary, iv or oral therapy with Tamiflu may be continued for up to 5 additional days. Anticipated time on study treatment is 5 to 10 days. 