Vaccinia virus is a large, complex, enveloped virus belonging to the poxvirus family. It has a linear, double-stranded DNA genome approximately 190 kbp in length, and which encodes approximately 250 genes. The dimensions of the virion are roughly 360 × 270 × 250 nm, with a mass of approximately 5-10 fg. Vaccinia virus is well known for its role as a vaccine that eradicated the smallpox disease, making it the first human disease to be successfully eradicated by science. This endeavour was carried out by the World Health Organization under the Smallpox Eradication Program. Post eradication of smallpox, scientists study Vaccinia virus to use as a tool for delivering genes into biological tissues.
In the early 21st century, due to concerns about smallpox being used as an agent for bioterrorism, there was renewed interest in studying the Vaccinia virus.