Transmission Electron Micrograph (TEM) of a Vaccinia virus. Vaccinia virus is normally confined to cattle, but is conveyed to humans through vaccination, thereby, imparting immunity to the smallpox virus. 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. Magnification: unknown.