In 1980, the World Health Organization declared smallpox eradicated from the world; the last known natural case had occurred in Somalia in 1977, and the United States had stopped routinely vaccinating its citizens in 1972. However, with increasing concerns regarding domestic and international terrorism, smallpox has resurfaced as a potential threat to global health. We review the direct and indirect modes of smallpox transmission and how patterns of transmission vary substantially, depending on the severity of circulating disease, vaccination status, environmental and socioeconomic factors, and the setting of an outbreak. We examine mechanisms for controlling outbreaks of disease and preventing further transmission in the event of an outbreak, with an emphasis on smallpox vaccination.