The purpose of this study is to evaluate an investigational smallpox vaccine, called IMVAMUNE®, with respect to safety and immune (body's defense system) response. Participants will include healthy adults, age 18 or older born after 1971, who have not had smallpox vaccine before. Volunteers were originally assigned to 1 of 5 groups. In July 2007, a hold was placed on the Dryvax® groups and the study was modified. Volunteers, numbering 197, will be assigned by chance to one of 3 groups to be vaccinated twice with IMVAMUNE® vaccine or placebo (inactive substance) in Groups A and B, or to receive a single vaccination with IMVAMUNE® or placebo in Group F. Volunteers will complete a memory aid (diary) for 15 days following vaccination. Blood samples will be collected. Volunteers may participate for up to 425 days.