Vaccinia virus particles and bacteria. Coloured scanning electron micrograph (SEM) of vaccinia virus particles (green). 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. There are approximately ten times as many bacterial cells in the human flora as there are human cells in the body, with the largest number of the human flora being in the gut flora, and a large number on the skin. Magnification: x12000 when printed at 10 centimetres wide.