Smallpox virus. Coloured transmission electron micrograph (TEM) of Orthopoxvirus variola viruses, the causative agent of smallpox. The Orthopoxvirus genus includes the most complex and largest animal viruses known: Orthopoxvirus variola measures approximately 250x200 nanometres. These viruses are also unique in that they are DNA viruses which replicate in the cytoplasm of host cells. Although smallpox has been eradicated as a disease other Orthopoxvirus viruses may still cause human infections which are characterised by blistery skin eruptions. Magnification: x50,000 at 6x4.5cm size.