The Director General of the World Health Organisation, Dr. Margaret Chan, says three to four million people could be infected with Zika in the Americas this year.
Speaking today at an emergency meeting of the WHO in Geneva, Switzerland, Dr. Chan says Zika has gone from a mild threat to one of alarming proportions.
Dr. Chan says she has established a Zika emergency team to respond to what she calls the explosive spread of the virus.
That team is due to meet on Monday to decide whether Zika should be treated as a global emergency.
The last time an international emergency was declared was for the Ebola outbreak in West Africa which has killed more than 11,000 people.
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.