This study will evaluate the safety and efficacy of both Dryvax® and the new cell-cultured vaccine (CCSV) in a comparative fashion. Across 3 cohorts, 150 vaccinia-naive volunteers will be randomly assigned to receive either CCSV (100 volunteers) or Dryvax® (50 volunteers) in a blinded fashion. Subjects will be followed closely for up to 6 months and a subgroup of volunteers will be followed up to 3 years in order to evaluate the duration of immunity following vaccination. Another cohort will enroll 100 vaccinia-experienced volunteers and randomly assign them to receive either CCSV (50 volunteers) or Dryvax® (50 volunteers) and a sub group will be followed up to 3 years. A fifth cohort will enroll 100 vaccinia-naive volunteers and randomly assign them to receive different dilutions of CCSV (1:1, 1:5, 1:25, and 1:50).