The material Jenner used for his vaccine probably contained cowpox virus, a virus related to variola but not as virulent. At some time during the 19th century, the virus used for smallpox vaccination ceased to be cowpox and changed to vaccinia. Vaccinia is in the same family as cowpox and variola, but is genetically distinct from both of them. The origin of vaccinia, and how it came to replace cowpox virus in the vaccine, is not known.