Animation of Zika virus particles in the bloodstream. This virus is spread by the bites of infected mosquitoes. It originated in tropical east Africa, and was first detected in Uganda in 1947. Since April 2015 there has been a significant outbreak in South America, which started in Brazil and is spreading rapidly. The World Health Organisation stated it expected the disease to have spread to the majority of the Americas by the end of 2016. There have been reports of sexual transmission of the virus, and also of birth defects in pregnant women who become infected, with a rise in the rate of microcephaly being linked to the disease. Zika virus is a flavivirus, related to yellow fever. At around 50 nanometres in diameter it is more than 100 times smaller than a red blood cell.