The World Health Organization announced Thursday that it will convene an emergency meeting to try to find ways to stop the transmission of the Zika virus — which officials said is “spreading explosively” across the Americas.

“The level of alarm is extremely high, as is the level of uncertainty. Questions abound. We need to get some answers quickly, ” Margaret Chan, director general of the WHO, said in Geneva in a briefing for member countries.

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

Health officials said 23 countries are affected by mosquitoes that are spreading the virus 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, the CDC’s Lyle Petersen said, and “this number is increasing rapidly.” The country also has 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, U.S. officials said all states here are now required to report Zika cases. As a result, they said they expect to see a sharp increase in reports of travel-related cases. But they reiterated that the United States is unlikely to have the kind of widespread local outbreaks that are taking place in other parts of the Americas.

Global health officials have come under criticism from some public health experts for not moving quickly enough to call an emergency meeting on Zika. They accused the WHO of failing to learn its lesson from the Ebola epidemic of 2014, when the agency delayed sounding the alarm for months. Several academic researchers wrote in the journal JAMA warning of Zika’s pandemic potential and urging the global health organization to take more urgent action.

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 risk of disease spread by mosquitos 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 any possible risk here in the United States.”

He pointed out that President Barack Obama convened a group of scientists and public health officials to discuss the efforts to combat the disease and said travel warnings were issued by the CDC and a public information campaign from CDC and NIH was launched.

“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 risk of the Zika virus are minimal.”

Brazil is the epicenter of Zika, and public health officials are investigating a link between the virus and a rare brain defect called microcephaly in infants, as well as a nervous system syndrome known as Guillain-Barre that can lead to paralysis.

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. Some of those have turned out not to be microcephaly, but many of them have been confirmed through ultrasound, he said. He did not provide a figure. Pregnant women who tested positive for the Zika virus and later had babies with microcephaly have had 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 the group’s position is that women who are pregnant should engage in “an abundance of caution” to avoid being bitten by mosquitoes.

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.”

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

The WHO said the reason Zika appears to be spreading so rapidly is twofold: One, because it is a new disease to the region, the population does not have immunity, and two, 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.

CDC deputy director Anne Schuchat told reporters Thursday that living conditions in the United States, such as better air conditioning and more window screens, 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.

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.

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 while some did not.

“We don’t have an answer as to what is actually going on,” he said.

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.

“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. 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.

“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 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 re-emergence 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 to fight the outbreaks.

David Nakamura contributed to this report.