Influenza, or the flu, is an infectious respiratory disease that can range in severity from mild to severe to even death. This study aims to evaluate a treatment for people who are hospitalized with the flu. The study is looking to see if antibodies collected from people who have recovered from the seasonal flu or who have had the seasonal flu shot can be used safely as a study drug to treat hospitalized patients with severe flu infections. Also, this study will help to find the right dose for this study drug for treatment of severe flu in hospitalized patients. Overall, this study will evaluate if the hospitalized patients receiving standard of care along with the study drug get better more quickly than those treated with standard of care and placebo. The study drug that contains antibodies against the flu is called anti-influenza immune globulin intravenous (FLU-IGIV).