This open-label study will assess the pharmacokinetics/pharmacodynamics 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.