The World Health Organization estimates there could be 3 to 4 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.

Declaring that the Zika virus is "spreading explosively," chief of the World Health Organization announced that it will hold an emergency meeting of independent experts on Monday to decide if the virus outbreak should be declared an international health emergency.

At a special meeting Thursday in Geneva, WHO Director-General Dr. Margaret Chan said the virus — which has been linked to birth defects and neurological problems — is becoming more of a threat.

Chan said although there was no definitive proof that the Zika virus was responsible for a spike in the number of babies being born with abnormally small heads in Brazil, "the level of alarm is extremely high." She also noted a possible relationship between Zika infection and Guillain-Barre syndrome, which can cause temporary paralysis.

"The possible links, only recently suspected, have rapidly changed the risk profile of Zika from a mild threat to one of alarming proportions. The increased incidence of microcephaly is particularly alarming, as it places a heart-breaking burden on families and communities," Chan said.

The Zika virus was first detected in 1947 and for decades only caused mild disease, but Chan noted that "the situation today is dramatically different." According to the U.S. Centers for Disease Control, the Zika virus is now in more than 20 countries, mostly in Central and South America. It is spread by the Aedes mosquito, which also spreads dengue and yellow fever.

WHO called the special session in part to convey its concern about an illness that has sown fear among many would-be mothers, who have responded by covering themselves head-to-toe in clothing in largely tropical Brazil or putting on many coats of insect repellent.

Chan cited four main reasons why WHO was "deeply concerned" about Zika: The possible link to birth defects and brain syndromes, the prospect of further spread, a lack of immunity among people living in the newly affected areas and the absence of vaccines, treatments or quick diagnostic tests for the virus.

Declaring a global emergency is akin to an international SOS signal and usually brings more money and action to address an outbreak. The last such emergency was announced for the devastating 2014 Ebola outbreak in West Africa, which eventually ending up killing over 11,000 people. Polio was declared a similar emergency the year before.

Still, convening an emergency committee does not guarantee that a global emergency will be declared — WHO has held 10 such meetings to assess the Middle Eastern respiratory syndrome coronavirus and no emergency has been announced.

One reason the U.N. health agency may be examining the Zika virus so quickly is because WHO was criticized for its slow response to Ebola; nearly 1,000 people had died before the agency declared it to an international emergency. The Associated Press found that senior agency officials resisted the Ebola declaration for two months, citing political and economic reasons.

Marcos Espinal, WHO's director of infectious diseases in the Americas region, said Brazil is conducting studies to determine if there is scientific evidence that Zika virus causes birth defects and neurological problems. He said they are hopeful Brazil may have data to share in a couple of months.

Brazil's Zika outbreak and the spike in microcephaly cases among babies have been concentrated in the poor and underdeveloped northeast of the country, though the prosperous southeast, where Sao Paulo and Rio de Janeiro are located, are the second hardest-hit region. Rio de Janeiro is of special concern, since it will host the Aug. 5-21 Summer Olympic games that are expected to be attended by millions from around the world.

Earlier this week, officials in Rio ramped up their fight against the mosquitoes that spread Zika, dispatching a team of fumigators to the Sambadrome, where the city's Carnival parades will take place next month.

There is no specific treatment or vaccine for Zika. The virus is related to dengue, and scientists have struggled for years to develop a dengue vaccine; the first such shot made by Sanofi Pasteur was licensed last year in Brazil.