The purpose of this study is to assess the safety of an experimental smallpox vaccine (MVA: Modified Vaccinia Ankara) and to compare the body's immune (system that fights disease) response to this vaccine. Participants will be assigned to 1 of 6 study groups. Each group will include 12 subjects, 10 will receive the modified smallpox vaccine and two will receive placebo, an inactive substance. The vaccine will be administered in 1 of 3 ways: under the skin; in the muscle; or between the muscle and the skin. Groups A and B will receive Dryvax® 6-15 months after the initial MVA vaccine; groups C, D, E, and F will receive Dryvax® 6 months after the initial MVA vaccine. Study procedures will include documenting side effects for 14 days after each vaccination, electrocardiogram (picture of the hearts activity) and blood samples. Participants will be involved in study related procedures for up to 18 months.