 This study will test the safety of an experimental vaccine called modified vaccinia virus ankara MVA and determine if it confers protection against the smallpox virus variola. There is an existing vaccine, called Dryvax Registered Trademark, which is effective against smallpox; however, this vaccine can cause various side effects, including some that, on rare occasions, can be life-threatening. Dryvax Registered Trademark has not been used in the United States since childhood vaccination was stopped in 1971, and though it is given to certain healthcare and laboratory workers, and some people in the armed forces, it is not recommended for the general population. Both the MVA and Dryvax Registered Trademark vaccines are made using the vaccinia virus, which is closely related to variola. 