 Study participants will be randomly assigned to one of the following dosing groups: 1 one injection of MVA; 2 one injection of placebo; 3 two injections of MVA 4 weeks apart; or 4 two injections of placebo 4 weeks apart. All participants will receive a challenge dose of Dryvax Registered Trademark 12 weeks after their last injection of MVA or placebo to determine if the MVA vaccine has conferred immunity. A take, that occurs in response to the Dryvax Registered Trademark dose indicates lack of prior immunity, and thus tells whether one or two doses of MVA is needed to produce an immune response. 