A member of the Flavivirus family, related to the viruses that cause yellow fever, West Nile disease, St Louis encephalitis, dengue, and Japanese encephalitis, Zika virus first was isolated from a rhesus monkey in the Zika forest region of Uganda in 1947. The first human cases of Zika virus disease were reported from Uganda and Tanzania in 1952. Only sporadic disease was observed during the next 55 years until 2007, when an outbreak was reported in Micronesia, the first time that Zika virus disease had been found outside Africa and Asia. Since then, the virus has appeared in a number of other locations across the South Pacific; in May 2015, the Ministry of Health of Brazil confirmed the first known transmission of Zika virus in Brazil and the Americas. As of February 2016, active Zika virus transmission was occurring in 26 countries in the Americas and the Caribbean, as well as in American Samoa, Samoa and Tonga in the South Pacific, and Cape Verde off the west coast of Africa. Brazil was the most affected country, with estimates between 440 000 and 1.3 million cases of Zika virus disease through December 2015.