The purpose of this randomized, double-blind, placebo-controlled trial of intravenous hyperimmune immunoglobulin (Flu-IVIG) in individuals with influenza A or B is to determine the pharmacokinetic (PK) profile of Flu-IVIG and assess whether antibody levels observed following Flu-IVIG transfusion are similar to those predicted. This pilot study will inform a larger study that will be powered to compare Flu-IVIG with placebo for efficacy.