False-colour transmission electron micrograph of vaccinia virus. A member of the poxvirus family, it is the largest & most complex of the vertebr- ate viruses (230x300 nanometres in size). The virus particles appear brick-shaped & consist of a complex structure of membranes surrounding the genetic core of DNA - the nucleoid. Replication of the DNA & formation of the 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 & humans that causes skin lesions resembling those of smallpox, but without the associated systemic disturbances. Magnification: x58,800 at 6x4.5cm size.