http://www.cdc.gov/ncidod/monkeypox/qa.htm
National Center for Infectious Diseases, Centers for Disease Control and Prevention
Monkeypox is caused by Monkeypox virus, which belongs to the orthopoxvirus group of viruses. Other members of this group of viruses that cause infections in humans include variola (smallpox), vaccinia (used for smallpox vaccine), and cowpox viruses.