 Participants will be observed for at least 1 hour after each injection. They will come to the clinic at least once a week after MVA or placebo injections and at least twice a week after Dryvax Registered Trademark to have the injection site evaluated and photographed. At each visit, participants will be asked how they are feeling and what medications, if any, they are taking. Blood and urine tests will be done according to the following schedule: 