In 1979, the World Health Organization declared that smallpox (variola) had been eradicated successfully worldwide. The last naturally occurring case of smallpox occurred in Somalia in 1977, followed by 2 cases attributable to laboratory exposure in 1978. The United States discontinued routine childhood immunization against smallpox in 1972 and routine immunization of health care professionals in 1976. The US military continued to immunize military personnel until 1995. Two World Health Organization reference laboratories were authorized to maintain stocks of variola virus. There is concern that the virus and the expertise to use it as a weapon of bioterrorism may have been misappropriated. As a result, the United States resumed the military immunization program in 2002 and initiated a civilian preevent smallpox immunization program in 2003 to facilitate preparedness and response to a smallpox bioterrorism event.