This study will compare the influenza A/H5N1 virus vaccine given by injection in the muscle versus injection in the skin in healthy adults. The study will look at the safety of the injections, how the body reacts, and what the body's immune response does when the vaccine is given in the muscle versus in the skin. The study will look at 226 healthy volunteers, ages 18-49 years old. Study procedures will include getting 2 doses of vaccine 28 days apart, physical exams, follow-up clinic visits to check the places on the body where each vaccine was given, and blood sample collections. Volunteers will complete a memory aid by writing down temperatures and health changes for 7 days after each vaccination. Volunteers will be involved in the study for up to 241 days.