A multi-centre, randomized, placebo controlled, trial. Participants will be patients either ≥65 years or with one or more high risk conditions presenting to one of four academic emergency departments in Edmonton or Calgary with influenza-like illness. The investigators will test for influenza using a point-of-care rapid test and if positive for influenza participants will be randomized to oseltamivir or placebo and followed prospectively. The primary outcome will be hospitalization.

Inclusion Criteria:

• Outpatient at the University of Alberta Hospital, the Northeast Community Centre, Foothills Medical Centre, Rockyview General Hospital emergency departments
• ≥ 18 years of age
• ≥ 65 years of age OR chronic cardiac or pulmonary disorders (including bronchopulmonary dysplasia, cystic fibrosis, and asthma) , diabetes mellitus and other metabolic diseases, cancer, immunodeficiency, immunosuppression (due to underlying disease and/or therapy), renal disease, anemia, and hemoglobinopathy, any condition that can compromise respiratory function or the handling of respiratory secretions or that can increase the risk of aspiration
• Acute respiratory infection (ARI) exhibiting 2 or more symptoms, onset within last 72 hours
• Laboratory confirmation of influenza infection, onset within last 72 hours

Exclusion Criteria:

• Contraindication to oseltamivir (i.e., previous anaphylaxis)
• Resident of a nursing home
• Canadian Triage and Acuity Scale (CTAS) 1
• Blood pressure < 90 mmHg
• Respiratory rate >30 breaths per minute
• PaO2 less <88%
• Confusion
• Inability to eat or drink
• Radiographic evidence of pneumonia