The purpose of this study is to assess the safety and immune response (body's defense against disease) to an experimental H1N1 influenza vaccine against the 2009 H1N1 virus. This study will help determine how the H1N1 flu shot should be given with the seasonal flu shot to make it most effective. Participants will include up to 850 healthy adults, ages 18 and older. Participants will receive 2 H1N1 vaccines in addition to placebo (inactive substance) and the seasonal flu shot over 3 study visits about 21 days apart. Study procedures include: medical history, physical exam, maintaining a memory aid, and blood sample collection. Participants will be involved in the study for about 8 months.