The WHO said the pathogen, which was virtually unheard of in the region a year ago, is spreading so fast that it could infect as many as 3 million to 4 million people within 12 months. Chan said those numbers and the severity of the possible complications being reported -- from a brain abnormality called microcephaly in children to paralysis in adults -- make the situation dramatically different than what epidemiologists have seen with past outbreaks of the virus.

Health officials said 24 countries and territories are affected by mosquitoes that are transmitting Zika locally. The Centers for Disease Control and Prevention said the United States has 31 confirmed cases in 11 states and the District of Columbia. All are travel-related, said Lyle Petersen, director of CDC’s vector-borne disease division, and "this number is increasing rapidly." At least one involves a pregnant woman, New York City officials said Thursday. There also are 20 additional cases because of local transmission in U.S. territories — 19 in Puerto Rico and one in the U.S. Virgin Islands.

In a separate briefing with reporters Thursday, U.S. officials said all states are now required to report Zika cases. As a result, they expect to see a sharp increase in cases involving a traveler infected while abroad who becomes symptomatic after returning home. But local outbreaks are unlikely here, officials said.

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Global health authorities have already been criticized for not moving quickly enough to call an emergency meeting on Zika. Some public health experts accused the WHO of failing to learn lessons from the Ebola epidemic of 2014, when the organization delayed sounding the alarm for months.

Lawrence O. Gostin, a global health law professor at Georgetown University, said Chan needs to "urgently mobilize international resources" to curb Zika's spread. "It is far better to be over-prepared than to wait until a Zika epidemic spins out of control," he said in a statement.

"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 added.

At his daily briefing, White House press secretary Josh Earnest was asked if the U.S. and international response to the Zika virus was too slow.

“What you have seen from this administration is a response consistent with the kind of threat that could be out there. At this point, here in the United States, the risks of disease spread by mosquitoes are quite low," he said. "The temperatures in North America right now are inhospitable to the mosquito population. Eventually that will change, and we have to be mindful of any possible risk here in the United States.”

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Earnest noted that President Obama on Tuesday convened a group of scientists and public health officials to discuss efforts to combat the disease and noted travel warnings issued by the CDC and a public information campaign by CDC and NIH.

“We are in a stage right now where we want to educate the public about what the risks are," he said. "For most people, the risks of the Zika virus are minimal.”

Brazil is the epicenter of Zika, and public health officials are investigating the suspected link between the virus and the rare condition that affects fetal brain development, as well as a possible association between the pathogen and, in adults, a syndrome known as Guillain-Barré that can lead to paralysis.

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During a briefing to the WHO executive board on Thursday, Brazil’s health minister, Claudio Maierovitch, said the country is investigating 12 confirmed deaths of babies born with microcephaly for potential linkage with Zika virus infection. The country has more than 4,000 suspected cases of microcephaly, and while some have turned out to be other conditions, many have been confirmed through ultrasound, he said. He did not provide a figure.

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Pregnant women who tested positive for the Zika virus and later had babies with microcephaly had experienced a rash and fever during the “first and second parts of their pregnancy,” he said.

Several countries in Latin America and the Caribbean have been so shaken by the reports that they have taken extreme measures by advising women of childbearing age to wait six months to two years before trying to become pregnant. Bruce Aylward, assistant director-general of the WHO, said its position is that women who are pregnant should engage in "an abundance of caution" to avoid being bitten by mosquitoes.

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Marcos Espinal, director of communicable diseases and health analysis for the Pan American Health Organization, said Zika is likely to spread to the same areas where dengue exists and predicted that “we can expect 3 to 4 million cases of Zika virus disease.”

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That reach includes parts of the southern United States, according to a map he presented at the briefing.

The WHO said Zika appears to be spreading so rapidly for two reasons: One, because it is a new disease to the region and so the population does not have immunity, and two, because the Zika virus is primarily transmitted by a mosquito species known as A. aegypti, which lives in every country in North and South America except Canada and Chile.

Previous outbreaks of Zika occurred primarily in areas with small populations -- a distinction from the situation in Brazil, where the virus is circulating in densely populated urban areas.

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CDC deputy director Anne Schuchat told reporters Thursday that living conditions in the United States, such as better air conditioning, more window screens and less crowding, are factors that make it much less likely for a widespread outbreak of Zika. But mosquito control is difficult even in this country, and she said state and local authorities need to be vigilant to "jump in" if there are locally transmitted cases.

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She said the Food and Drug Administration is also looking into whether any additional guidance is necessary regarding blood donations. Zika virus stays in the blood for only a few days, and "most people have cleared it by about a week," she said.

The FDA is assessing whether travelers who have visited affected regions should defer donating their blood, a spokeswoman said. The agency is also working on recommendations to help maintain a safe blood supply in U.S. territories where the virus is present. "We cannot speculate on specific implementation timing at this point," said FDA spokeswoman Tara Goodin.

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WHO officials said that this type of mosquito also has been simultaneously carrying a host of other viruses — dengue, Chikungunya, yellow fever, West Nile — to those regions in recent years. Among the hypotheses scientists are looking into are whether the recent severe reactions may be related to co-infection with Zika and another virus, or previous exposure to one.

Aylward said some of the women who gave birth to children with microcephaly had been tested, and some of them had other infections. "We don't have an answer as to what is actually going on," he said.

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There has been one reported case that the virus could have been transmitted through sex, and another case in the medical literature where the virus was found in semen two weeks after symptoms of infection, according to the CDC's Schuchat.

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"But the science is clear that Zika is primarily transmitted to people through the bite of an infected mosquito," she said. "That is really where we are putting our emphasis."

Part of the challenge with Zika is that it is often "silent," with up to 75 percent of infected patients having no symptoms, said Sylvain Aldighieri, who works in epidemic alert and response for the WHO/PAHO. "We have big gaps in terms of confirmation of the real situation."

Representatives from several countries raised concerns about whether we’re seeing a potentially more virulent mutated virus in the Americas, but WHO officials said tests so far show that it’s “very similar” to what was circulating in the Pacific region several years ago.

WHO officials said that better diagnostic tests are in the works, as well as possible antiviral therapies and vaccines, but that any of these could take months to develop. Meanwhile, efforts are focused on controlling the spread of the virus by eliminating mosquito populations.

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In some countries, health officials have been going door to door to spray for mosquito breeding grounds and have launched public education campaigns to urge people to wear repellent clothing or use sprays. In a controversial experiment, a British company has announced it would release genetically modified mosquitoes whose larvae don't make it to adulthood to see if they can help stop the spread of the virus.

Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said Thursday that investigators may be able to start a clinical trial for a vaccine as early as this year.

The WHO's Chan urged “every community, every family and individual” to do their part by, for example, taking care not to leave stagnant collections of water on their properties. She emphasized that every person in the world could be vulnerable to the virus.

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“The mosquito is ubiquitous,” she said. “You don’t need to travel to get the disease.”

The WHO special session on Zika is scheduled to take place on Monday, and delegates will discuss whether to declare it a global public health emergency -- a designation that could help mobilize a more coordinated response. The WHO has only done this three times before: in 2009 during the H1N1 influenza epidemic, in August 2014 with Ebola and in May 2014 regarding the reemergence of polio.

The declaration typically comes with a list of global recommendations to nations regarding everything from international travel and trade to scientific targets for diagnosis and treatment of a disease and has been considered critical to convincing wealthier countries to send more health workers and supplies to fight the outbreaks.

David Nakamura contributed to this report.

This post has been updated.