Participants will be observed for at least 1 hour after each injection. They will come to the clinic a week after MVA or placebo injections and at least twice a week after Dryvax® (Registered Trademark) for about 21 days to have the injection site evaluated and photographed. At each visit, participants will be asked about how they are feeling and if they are taking any medications. Blood and urine tests will be done on injection days and at follow up visits scheduled 1 and 4 weeks after each immunization as well as 12 weeks after the Dryvax® (Registered Trademark) challenge dose. Additional tests may be done between visits if medically necessary.