Phase I/II, randomized, placebo-controlled, double-blinded, schedule finding study of MVA. The hypothesis is that MVA will be safe in humans when administered by intramuscular (IM) injection and will result in an immune response comparable to that observed after Dryvax primary vaccination. The primary objectives include: evaluating the safety of MVA administered by intramuscular (IM) injection on single and multidose schedules and identifying a schedule of MVA that provides clinical evidence of protection against vaccinia (Dryvax) challenge. The secondary objectives are to compare the immunogenicity of Dryvax and MVA as measured by vaccinia-specific neutralizing antibody, ELISA, and intracellular cytokines assays and by a variola-specific neutralizing antibody assay that will be conducted at the CDC on a subset of samples.