Vaccinia virus particles. Coloured transmission electron micrograph (TEM) of sectioned vaccinia virus particles. The genetic cores (red) are covered by membrane layers (green) taken from the host cell that replicated the virus. Vaccinia belongs to the orthopoxvirus group. Formation of new virus particles occurs in the cytoplasm of the host cell, 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 Edward Jenner in 1796 to protect (vaccinate) against smallpox, a related and far more deadly human disease.