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Scientists have not studied variola virus well because of the hazards associated with potential exposure. In addition, by international agreement, smallpox may only be studied at the Centers for Disease Control and Prevention high-containment facility or one in Russia. Experiments must be approved in advance by an international committee.

Vaccinia virus, however, used to make a smallpox vaccine and closely related to variola, has been studied thoroughly. There is one major difference between the two viruses: vaccinia can infect several types of living beings, while variola infects only humans naturally and cynomolgus monkeys under highly artificial laboratory conditions.

Last Updated August 20, 2008