 The purpose of this research study is to compare how the body reacts to different strengths of the new H5N1 flu vaccine. The study will also look at how antibodies are made after a subject receives the H5N1 flu vaccine. The vaccine will be given as a shot in the arm. Participants will be randomly assigned to receive 1 of 4 different dosages of the vaccine or a placebo dummy dose of the vaccine. Study procedures will include a physical exam, a urine pregnancy test for women, and blood samples. Participants will be asked to complete a diary to record temperatures, medications taken, and side effects from the vaccine. Volunteers will be involved with study procedures for approximately 7 months. The study will enroll about 450-470 healthy participants, ages 18-64. 