Coloured transmission electron micrograph (TEM) of Zika virus particles (orange) isolated in kidney epithelial cells (Vero E6 cells). Zika viruses have a dense core surrounded by an envelope (40nm size). Zika virus is a RNA (ribonucleic acid) virus in the family Flaviviridae and causes Zika fever, or Zika disease. Aedes aegypti mosquitoes can transmit Zika virus to humans. Zika fever was first discovered in equatorial Africa and Asia in the late 1940s. From 2014-16 Zika fever has spread to South America. It has recently spread to the islands in the Caribbean. There is a possible link between Zika fever and microcephaly in newborn babies of mothers who have been infected with the Zika virus. In February 2016 Zika virus was discovered to be sexually transmitted in the USA. Magnification: x191, 200 when shortest axis printed at 25 millimetres. Image courtesy of Cynthia S Goldsmith/CDC.