The Zika virus is spread by the Aedes aegypti mosquito, which also carries dengue fever and yellow fever.
It was first discovered in Africa in the 1940s but is now spreading in Latin America
The Zika virus can lead to fever and a rash but most people show no symptoms, and there is no known cure.
The only way to fight Zika is to clear stagnant water where mosquitoes breed, and protect against mosquito bites.
Zika was first detected in Uganda in 1947, but has never caused an outbreak on this scale.
Brazil reported the first cases of Zika in South America in May 2015.
Most cases result in no symptoms and it is hard to test for, but WHO officials said between 500,000 and 1.5 million people had been infected in Brazil.
The Zika virus has since spread to more than 20 countries in the region.