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 known to cause severe disease in healthy individuals, but it can lead to complications in those with weakened immune systems. The virus is primarily used in research and vaccine development due to its ability to stimulate a strong immune response. It has also been studied for potential use in gene therapy and as a vector for delivering vaccines against other diseases, such as HIV and cancer.