Smallpox is caused by the double-stranded DNA orthopoxviruses Variola major and Variola minor. The virus no longer occurs naturally. Due to an aggressive immunization program by the World Health Organization and its inability to survive for more than a week outside of its only host, humans, Variola was declared eradicated in 1980. The two known stock of the virus are at the U.S. Center for Disease Control and Prevention in Atlanta and the State Research Center of Virology and Biotechnology in Koltsovo, Russia. Rumors are that other countries may have retained their stock of smallpox and the Russian stock may have been compromised.