Vaccinia virus is a member of the Poxviridae family and is best known as the virus used in the smallpox vaccine. It is a large, double-stranded DNA virus that can infect various animals, including humans. Vaccinia virus is not typically associated with severe disease in healthy individuals, but it can cause mild skin lesions and flu-like symptoms. The virus is important in the history of vaccination, as it helped eradicate smallpox, a deadly disease that affected millions. Vaccinia virus is also studied for its potential use in gene therapy and as a vector for delivering vaccines against other diseases.