Oseltamivir is approved for prophylaxis and treatment of children 1 year of age and older with influenza. Influenza treatments for children under the age of 1 year are needed because mortality from influenza is high among this age group, even when there are no underlying medical conditions. Oseltamivir is frequently used off-label in children less than 1 year of age, with no data supporting the doses being used. Given the risk of severe or fatal influenza infection in infants, the lack of repeat dose pharmacokinetic (PK) data in children less than 2, the need for treatments in this population of children, and the fact that oseltamivir is being used off-label in this population, the current study will systematically study the PK and safety of oseltamivir in children less than 2 years of age with confirmed influenza to determine the appropriate dose to be used in these age groups. This data will be critical to pediatricians caring for these potentially gravely ill infants. This study is a prospective, age-stratified PK/pharmacodynamic (PD) and safety evaluation of oseltamivir therapy in children less than 24 months of age with confirmed influenza infection. Participants will be stratified by age into the following enrollment scheme at study initiation: 12-23 months (Cohort I), 9-11 months (Cohort II), 6-8 months (Cohort III), 3-5 months (Cohort IV) and 0-2 months (Cohort V). At study onset, Cohort II and III will be enrolled simultaneously. Cohorts IV and V will be enrolled sequentially by decreasing age groups predicated upon the PK and safety data from the preceding cohort. In the event of a public health emergency, the Data Safety Monitoring Board (DSMB) or Food and Drug Administration (FDA) may authorize the following modifications to the proposed enrollment plan: the opening of younger age cohorts without the full dataset from the next higher age cohort, the re-opening of previously closed cohorts to obtain additional data and/or the over-enrollment of any of the 5 cohorts. The oldest cohort (Cohort I) may be enrolled at any time during the study. The primary study objective is to define the PK of oseltamivir and oseltamivir carboxylate in children with confirmed influenza less than 2 years of age. The oseltamivir dose initially evaluated in Cohort I was the approved dose of 30 mg twice a day (bid). However, the oseltamivir carboxylate area under the curve (AUC)12 values for 5 of the 9 subjects enrolled in Cohort I as of August 5, 2009, were below the lower range utilized for the other cohorts in the study, as was the GM AUC12 for Cohort I as a group \[(2589 nanograms per hour per milliliter (ngxh/mL)\]. As a consequence, the DSMB recommended on August 5, 2009, that the protocol be amended to utilize weight-based dosing of oseltamivir in subjects subsequently enrolled in Cohort I, and to employ the targeted AUC approach used for Cohorts II-V for this cohort as well. Based upon the PK data available as of that date, the initial weight-based dose to be evaluated for Cohort I is 3.5 mg/kg bid. A dose of oseltamivir 3 mg/kg/dose orally bid for 5 days (10 doses) will be administered to the first 9 subjects in each of Cohorts II-III. Additional subjects may be enrolled if the target AUC12 range is not achieved. The proposed dose for subjects enrolled in Cohorts IV and V will be 3 mg/kg/dose orally bid for 5 days (10 doses), although this dose may be adjusted prior to opening Cohort IV or V based on the dose required to achieve the target oseltamivir carboxylate AUC12 range in the previous cohort.