The World Health Organization on Thursday called an emergency committee on the Zika virus, which is "spreading explosively" and suspected of causing birth defects.
The meeting, scheduled for Monday, will examine whether the Zika outbreak should be classified as an international health emergency, WHO said in a statement.
WHO's Director General Dr. Margaret Chan said the virus is "spreading explosively" in the Americas. "The level of alarm is extremely high," she said.
Experts strongly suspect that Zika is causing a severe birth defect called microcephaly, in which babies' brains are underdeveloped. It's not certain yet, but evidence is building.
'The possible links, only recently suspected, have rapidly changed the risk profile of Zika, from a mild threat to one of alarming proportions," Chan said in remarks to WHO's executive board.
"The increased incidence of microcephaly is particularly alarming, as it places a heart-breaking burden on families and communities." But WHO officials stressed there were still doubts about what might actually be causing the rise in microcephaly in Brazil and note a similar increase in cases has not been seen in other countries that have or have had Zika.
At least 23 countries have local spread of Zika and WHO says the virus will likely eventually spread to almost every country in the Americas except Canada and Chile, which don't have the mosquito species that transmits the virus. WHO predicts the virus will infect 3 million to 4 million people in the Americas.
But U.S. health experts downplayed the risk to most U.S. residents. "For the average American who is not traveling, this is not a problem," Dr. Anne Schuchat of the U.S. Centers for Disease Control and Prevention told reporters. That's because it take two factors for the virus to spread: actively infected people and the right kind of mosquitoes to spread it.
The Aedes aegypti mosquitoes that spread Zika and related viruses such as dengue only circulate widely in very southern parts of the U.S. and Hawaii. "We do think the living conditions in general in the United States, the lasck of density, better air conditioning, wider use of screens, will keep us in better shape," said Dr. Tony Fauci, head of the National Institute of Allergy and Infectious Diseases.
WHO's assistant director general, Dr. Bruce Aylward, said the spread of the virus has not reached the highest level of alert for the organization. "Alarmed would not be the right language," Aylward told reporters."I think concerned would be the right language to use."
The main goal is to get out some coherent travel advice for people, Aylward said. He said it would be very unlikely that WHO would recommend against all travel to Brazil. Such a recommendation would be devastating to Brazil's economy.
Related: Five things to Know About Zika
WHO can also set a research agenda and help direct any needed aid efforts. Aylward said this will be the first big test of a new central emergency response mechanism set up after WHO admitted it had failed to respond quickly enough to the Ebola epidemic in West Africa.
On Wednesday, two experts on international health matters accused Chan and WHO of acting far too slowly in raising the alarm about Zika. They welcomed WHO's decision.
"The Director-General has taken a critical first step in recognizing the seriousness of an emerging epidemic," said Dr. Lawrence Gostin, who heads the O'Neill Institute for National and Global Health Law at Georgetown University.
"She now must urgently mobilize international resources to curb the rapid spread of Zika worldwide, including aggressive mosquito control, active surveillance, accelerated vaccine research and travel advisories for pregnant women. It is far better to be over-prepared than to wait until a Zika epidemic spins out of control."
Until last year, the virus had not been a major concern for health officials, causing mostly mild symptoms in around 20 percent of those infected.
"The situation today is dramatically different," Chan said. "The level of concern is high, as is the level of uncertainty... Questions abound. We need to get some answers quickly."
Zika is also suspected of causing other serious conditions, such as other brain damage to unborn babies besides microcephaly, as well as a paralyzing condition called Guillain-Barre syndrome.
There's no treatment for Zika, which causes no symptoms in 80 percent of people who get it. There's no vaccine, either. It wasn't suspected of causing birth defects until Brazil raised the alert in November.
The virus is carried by the same Aedes aegypti mosquitoes that carry dengue and yellow fever viruses and is expected to spread everywhere those mosquitoes are found. That includes most tropical regions as well as the southern U.S. and Hawaii.
"Moreover, conditions associated with this year's El Nino weather pattern are expected to increase mosquito populations greatly in many areas," Chan said.
"Questions abound. We need to get some answers quickly."
It will be imperative to protect unborn babies, said Carissa Etienne, who directs the Pan American Health Organization.
"Although we are still working to establish causality with Zika, we cannot tolerate the prospect of more babies being born with neurological and other malformations, and more people facing the threat of paralysis due to Guillain-Barré syndrome," Etienne told the WHO executive board meeting.
"If the association between microcephaly and Zika virus is confirmed, there will be an ethical imperative to protect women of childbearing age from contracting the infection," Gostin said in a statement. "The public will demand well-funded, proactive leadership from the World Health Organization."