False-colour transmission electron micrograph (TEM) of a vaccinia virus particle, a member of the orthopoxvirus group. Virus particles (virions) of this group appear brick-shaped; complex membrane structures surround the nucleoid, a genetic core of DNA. Replication of the 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.