This study will test the safety of an experimental vaccine called Modified Vaccinia Virus Ankara (MVA) for use against the smallpox virus. It will also investigate how many injections of MVA are needed to produce immunity against vaccinia virus, which is closely related to the smallpox virus. An effective smallpox vaccine exists, but it can cause side effects that, on rare occasions, can be life-threatening. The FDA gave new license approval for Dryvax on 10/25/02, but has not been used in the general population since smallpox was eradicated worldwide. Both the MVA and Dryvax® (Registered Trademark) vaccines are made using the vaccinia virus, however the MVA vaccine contains a more attenuated, or weakened, form of the virus. [http://www.fda.gov/cber/products/smalwye102502.htm]