Vaccinia virus particles. Coloured transmission electron micrograph (TEM) of a section through vaccinia virus particles (virions). Vaccinia belongs to the orthopoxvirus group, whose genetic information is in the form of DNA. Formation of viral progeny occurs in the host cell's cytoplasm, whereas most other viruses replicate in the cell nucleus. The vaccinia virus causes cowpox, a disease of cattle and humans, which produces skin lesions. It was first used by Jenner in 1796 to protect against smallpox, a related but more deadly human disease, in the process of vaccination. Magnification unknown.