False-colour transmission electron micrograph (TEM) of vaccinia virus particles. The 3 Virus particles (virions) visible appear brick-shaped, with a complex surface. Vaccinia belongs to the orthopoxvirus group; genetic information is in the form of DNA. Formation of viral progeny occurs in the host cell's cytoplasm; most other viruses replicate in the cell nucleus. The vaccinia virus causes cowpox, a disease of cattle. It was first used by Jenner in 1796 to protect against smallpox, a related human disease, in the process of vaccination. Magnification: x16,000 at 6x7cm size.