The purpose of this study is to gather information on the safety and the effectiveness of an investigational vaccine for the prevention of smallpox disease. Smallpox was one of the major causes of death and sickness through the first half of the 20th century, but a global program of smallpox eradication resulted in the elimination of the natural disease. The last cases of smallpox in the United States occurred in 1949 in Texas. Today, only laboratory workers who work with smallpox-related viruses, military personnel, and health care workers are vaccinated.