Vaccinia virus particles. Coloured scanning electron micrograph (SEM) of vaccinia virus particles replicating. The nucleocapsids (coats, green) have split and DNA (deoxyribonucleic acid, orange) has been released into the infected cell's cytoplasm. This is unusual as most other viruses replicate in the host cell's nucleus. The yellow structures are the cell's golgi bodies, an organelle that the virus uses to make its envelope. Vaccinia belongs to the orthopoxvirus group. It causes cowpox, a disease of cattle and humans, which produces skin lesions. It was first used by Jenner in 1796 to vaccinate against smallpox, a related but more deadly human disease. Magnification: x54900 when printed 10cm wide.