Vaccinia virus particles. Coloured scanning electron micrograph (SEM) of vaccinia virus particles (blue) budding from a cell (yellow) after replicating. Unlike most viruses, vaccinia replicates in the cell's cytoplasm, rather than the nucleus. Vaccinia belongs to the orthopoxvirus group. It causes cowpox, a disease of cattle and humans that produces skin lesions. It was first used by Jenner in 1796 to vaccinate against smallpox, a related but more deadly human disease. Magnification: x5000 when printed at 10 centimetres wide.