The overall goal of this study is to study influenza vaccine responses in HIV infected individuals. Immunocompromised individuals require special protection from influenza, but may not respond appropriately to the standard killed vaccine. Patients who receive the H1N1 flu vaccine as part of their standard of care will be asked to donate blood samples for immunologic studies. These studies will determine whether participants were able to produce the appropriate antibodies to the vaccine and possibly identify predictors of vaccine responsiveness.