This study is will gather critical information on the safety, tolerability, and the immunogenicity (capability of causing an immune response) of A/H5N1 (Bird flu) virus vaccine in healthy adults. Up to 400 healthy adults, aged 18 to 64, will participate. Each subject will participate for about 7 months and will be randomly placed in one of several different study groups receiving a different dose of vaccine or placebo. All subjects will receive two injections of their assigned study product, about 28 days apart, in the muscle. Subjects will keep a journal of their temperature and any adverse effects between study visits. A small amount of blood will also be drawn before the first injection, 7 days after each injection, 1 month after the first injection, and 1 and 6 months after the second injection.