The Zika virus may infect 3-4 million people in the Americas, including 1.5 million in Brazil, the World Health Organisation has said.
The mosquito-borne Zika virus, which is thought to cause defects in newborn babies, is now "spreading explosively", the body said.
Britain's camp for the Rio Olympics should be moved to a Zika-free location outside Brazil, Jessica Ennis-Hill's coach has said.
The mother of Zika "bucket baby" Jose Ferreira is holding out hope despite doctors' fears over his future.
The government has told airlines to take the measure on all flights travelling to the UK from countries with confirmed transmission of Zika.