The purpose of this study is to assess the safety and the body's immune response (body's defense against disease) to an experimental H1N1 influenza vaccine in healthy adult and elderly populations. The study will enroll up to 450 healthy adults ages 18 and older with no history of H1N1 infection or vaccination. Two hundred individuals will be 18-64 years old, and the other 200 will be greater than or equal to 65 years of age. Participants will be randomly assigned to 1 of 2 possible vaccine groups: group 1 will receive 15 mcg of H1N1 vaccine; group 2 will receive 30 mcg of H1N1 vaccine. Both groups will receive vaccine injections on days 0 and 21 in the arm muscle. Study procedures include: medical history, physical exam, maintaining a memory aid, and blood sample collection. Participants will be involved in study related procedures for approximately 7 months.