Most people with Zika don't show signs - WHO
The World Health Organization estimates there could be three to four million cases of Zika in the Americas over the next year.
Sylvain Aldighieri, head of WHO's epidemic response team in the Americas, said the estimate is based on previous numbers of infections of dengue fever, which is also carried by mosquitoes.
He said the agency expects "huge numbers" of infections because of the widespread presence of the mosquitoes that spread Zika and because there is no immunity among the population.
He said that since most people with Zika don't get sick, there is a "silent circulation" of the disease that may make tracking its spread more difficult
Zika is suspected of being behind the birth of babies with abnormally small heads.