False-colour transmission electron micrograph of vaccinia virus. It is a member of the poxvirus family, the largest & most complex of the vertebr- ate viruses (230x300 nanometres in size). The virus particles appear brick-shaped & are made of a complex structure of membranes surrounding the genetic core of DNA - the nucleoid. Replication of the DNA & the formation of the viral progeny occurs in the host cell's cytoplasm; most other viruses replicate within 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: x17,500 at 35mm size. Orange tint. Original is BW print M050/091.