The World Health Organization (WHO) predicts that three to four million people could be infected with Zika in the Americas this year.
Most will not develop symptoms, but the infection has been linked to brain defects in babies.
WHO director general, Dr. Margaret Chan said Zika had gone “from a mild threat to one of alarming proportions”. She has set up a Zika “emergency team” after the “explosive” spread of the virus.
The last time an international emergency was declared was for the Ebola outbreak in West Africa which has killed more than 11,000 people.