Vaccinia virus (VACV or VV) is a large, complex, enveloped virus belonging to the poxvirus family. It has a linear, double-stranded DNA genome approximately 190 kbp in length, which encodes approximately 250 genes. The dimensions of the virion are roughly 360 × 270 × 250 nm, with a mass of approximately 5–10 fg.Smallpox was the first disease to be widely prevented by vaccination, due to pioneering work by the English physician and scientist Edward Jenner, in the eighteenth century, using cowpox virus. Vaccinia virus is the active constituent of the vaccine that eradicated smallpox, making it the first human disease to be eradicated. This endeavour was carried out by the World Health Organization under the Smallpox Eradication Program. Following the eradication of smallpox, scientists study vaccinia virus to use as a tool for delivering genes into biological tissues (gene therapy and genetic engineering) and because of concerns about smallpox being used as an agent for bioterrorism.