(CNN) The Zika virus "is now spreading explosively" in the Americas, the head of the World Health Organization said Thursday, with another official estimating between 3 million to 4 million infections in the region over a 12-month period.

"The level of concern is high, as is the level of uncertainty," Dr. Margaret Chan, WHO's director-general, told her organization's executive board members. "We need to get some answers quickly."

The lack of any immunity to Zika and the fact that mosquitoes spreading the virus can be found most "everywhere in the Americas" -- from Argentina to the southern United States -- explains the speed of its transmission, said Dr. Sylvain Aldighieri, an official with the WHO and Pan American Health Organization.

Aldighieri gave the estimate for Zika infections (including people who do not report clinical symptoms) based on data regarding the spread of a different mosquito-borne virus -- dengue. He acknowledged the virus is circulating with "very high intensity."

Some 80% of those infected with the Zika virus don't even feel sick, and most who do have relatively mild symptoms such as a fever, rash, joint pain or pink eye. But there are major worries about the dangers pregnant women and their babies face.

Chan said that, where the virus has arrived, there's been a corresponding "steep increase in the birth of babies with abnormally small heads and in cases of Guillain-Barre syndrome ." Having small heads can cause severe developmental issues and sometimes death. Guillain-Barre is a rare autoimmune disorder that can lead to life-threatening paralysis.

The WHO's Dr. Bruce Aylward cautioned there was no definitive link between Zika and these disorders but sees a legitimate reason for concern. The Centers for Disease Control and Prevention's Dr. Anne Schuchat said there is a "strong" suggestion they are connected.

While studies are underway to determine any links, millions of people live in areas with real fears about what this virus can do.

Pregnant women, their babies at high risk

After first being detected in 1947 in a monkey in Uganda, Zika was most often found along the equator from Africa into Asia. Nine years ago, new cases popped up in islands in the Pacific Ocean.

Last year, the virus made its way to the Americas -- with devastating results.

Since Brazil made its first discovery of Zika in May, the number of cases there and elsewhere in the Americas has grown exponentially. The virus had been thought to be relatively harmless over the long term, but that view changed late last year.

Health authorities began to suspect a connection between Zika and neurological ailments, especially in fetuses and newborns. Brazil alone has reported more than 4,000 cases of microcephaly -- a neurological disorder resulting in the births of babies with small heads -- in infants born to women infected with Zika while pregnant.

"Zika is not a new virus," the CDC's Schuchat said. "But what we are seeing in the Americas is new."

Photos: Zika virus outbreak A pest control worker fumigates a school corridor on the eve of the annual national Primary School Evaluation Test in Kuala Lumpur on Sunday, September 4. Malaysia reported its first locally transmitted Zika case on September 3. Hide Caption 1 of 29 Photos: Zika virus outbreak A banner about Zika virus is seen as ferry passengers arriving from Singapore get in line at the immigration check on September 4, in Batam, Indonesia. Hide Caption 2 of 29 Photos: Zika virus outbreak A banner is flown over the South Pointe Park area, Tuesday, September 6, in Miami Beach, Florida. Hide Caption 3 of 29 Photos: Zika virus outbreak A pest control worker fumigates drains at a local housing estate where the latest case of Zika infections were reported on Thursday, September 1, in Singapore. Hide Caption 4 of 29 Photos: Zika virus outbreak Florida Gov. Rick Scott and Florida Surgeon General Dr. Celeste Philip address the media gathered at the Miami-Dade County Department of Health as they announce five cases of Zika in a 1.5 mile area of Miami Beach on Friday, August 19, in Miami, Florida. Hide Caption 5 of 29 Photos: Zika virus outbreak Brazil's interim President Michel Temer, center right, meets with officials during Temer's first visit to the Olympic Park on Thursday, June 14, in Rio de Janeiro. The Rio 2016 Olympic Games commence August 5 amid a political and economic crisis in the country along with the Zika virus outbreak. Hide Caption 6 of 29 Photos: Zika virus outbreak Rio 2016 Chief Medical Officer Joao Grangeiro, Municipal Secretary of Health Daniel Soranz and Sub-secretary of the State for Health and Surveillence Alexandre Chieppe field questions from the media during an International Media Briefing to address the Zika virus on Tuesday, June 7, in Rio de Janeiro. Hide Caption 7 of 29 Photos: Zika virus outbreak A health worker fumigates an area in Gama, Brazil, to combat the Aedes aegypti mosquito on Wednesday, February 17. The mosquito carries the Zika virus, which has suspected links to birth defects in newborn children. The World Health Organization expects the Zika outbreak to spread to almost every country in the Americas. Hide Caption 8 of 29 Photos: Zika virus outbreak A man places a mosquito net over a bed at a home for the elderly in Masaya, Nicaragua, on Thursday, February 11. Hide Caption 9 of 29 Photos: Zika virus outbreak An Aedes aegypti mosquito floats in stagnant water inside a tire at a used tire store in Villavicencio, Colombia, on Thursday, February 4. Hide Caption 10 of 29 Photos: Zika virus outbreak A health worker fumigates an area in Caracas, Venezuela, on Tuesday, February 2. Hide Caption 11 of 29 Photos: Zika virus outbreak A lab worker exposes his arm to Aedes aegypti mosquitoes during testing at the Roosevelt Hospital in Guatemala City, Guatemala, on Monday, February 1. Hide Caption 12 of 29 Photos: Zika virus outbreak Tainara Lourenco sits inside her home in Recife, Brazil, on Friday, January 29. Lourenco, five months pregnant, lives at the epicenter of Brazil's Zika outbreak. The Zika virus has been linked to microcephaly, a neurological disorder that results in newborns with small heads and abnormal brain development. Hide Caption 13 of 29 Photos: Zika virus outbreak Dr. Angela Rocha shows brain scans of a baby born with microcephaly at the Oswaldo Cruz Hospital in Recife on Thursday, January 28. Hide Caption 14 of 29 Photos: Zika virus outbreak Alice Vitoria Gomes Bezerra, a 3-month-old baby with microcephaly, is placed in her crib by her father Wednesday, January 27, in Recife. Hide Caption 15 of 29 Photos: Zika virus outbreak A health ministry employee fumigates a home in Soyapango, El Salvador, on January 27. Hide Caption 16 of 29 Photos: Zika virus outbreak A lab technician at the Oswaldo Cruz Foundation Institute stores Aedes aegypti mosquitoes to be used in research in Recife on January 27. Hide Caption 17 of 29 Photos: Zika virus outbreak A patient suffering from Guillain-Barre syndrome recovers at a hospital in San Salvador, El Salvador, on January 27. Researchers are looking into a possible link between Zika and Guillain-Barre, a rare disorder that causes the body's immune system to attack its nerves. Hide Caption 18 of 29 Photos: Zika virus outbreak Luiz Felipe lives in Recife and is one of more than 4,000 babies in Brazil born with microcephaly since October. The drought-stricken impoverished state of Pernambuco has been the hardest-hit, registering 33% of recent cases. Hide Caption 19 of 29 Photos: Zika virus outbreak A health worker sprays insecticide under the bleachers of Rio de Janeiro's Sambadrome on Tuesday, January 26. Hide Caption 20 of 29 Photos: Zika virus outbreak A Brazilian soldier inspects a home in Recife on Monday, January 25, while canvassing the neighborhood and attempting to eradicate the larvae of mosquitoes linked to the virus. Hide Caption 21 of 29 Photos: Zika virus outbreak David Henrique Ferreira, a 5-month-old who has microcephaly, is watched by his brother in Recife on January 25. Hide Caption 22 of 29 Photos: Zika virus outbreak The larvae of Aedes aegypti mosquitoes are photographed in a lab in Cali, Colombia, on January 25. Scientists are studying the mosquitoes to control their reproduction and resistance to insecticides. Hide Caption 23 of 29 Photos: Zika virus outbreak Angelica Prato, a pregnant woman infected by the Zika virus, receives medical attention at a hospital in Cucuta, Colombia, on January 25. Hide Caption 24 of 29 Photos: Zika virus outbreak A woman walks through fumes as health ministry employees fumigate an area in Soyapango on Thursday, January 21. Hide Caption 25 of 29 Photos: Zika virus outbreak Brazilian soldiers apply insect repellent as they prepare for a cleanup operation in Sao Paulo on Wednesday, January 20. Hide Caption 26 of 29 Photos: Zika virus outbreak A researcher at the University of Sao Paulo holds a container with female Aedes aegypti mosquitoes on Monday, January 18. Hide Caption 27 of 29 Photos: Zika virus outbreak A graveyard in Lima, Peru, is fumigated on Friday, January 15. Hide Caption 28 of 29 Photos: Zika virus outbreak Aedes aegypti mosquitos are seen at the University of Sao Paulo on January 8. Researchers from the Pasteur Institute in Dakar, Senegal, came to Brazil to train local researchers to combat the Zika virus epidemic. Hide Caption 29 of 29

The mosquito-borne disease is in 23 countries and territories in the Americas, according to Chan.

There have been 32 documented cases in 12 states and the District of Columbia, though all of those people got infected in other countries. (There have been 19 laboratory-confirmed cases in Puerto Rico and one in the U.S. Virgin Islands.)

The states where Zika virus has been confirmed among travelers returning from affected countries are Arkansas, California, Florida, Hawaii, Illinois, Massachusetts, Minnesota, New Jersey, New York, Oregon, Texas, Virginia and the District of Columbia, according to health departments.

Schuchat expects the number of travel-associated U.S. cases to rise and for people to contract the disease from mosquitoes here (though she downplayed widespread transmissions).

There will likely be more outside the United States as well.

"We expect more countries to be affected," Schuchat said.

WHO calls emergency meeting

Chan has called an emergency committee meeting Monday in Geneva, Switzerland, to address the Zika virus' spread and its ramifications.

The gathering will aim to determine the appropriate "level of international concern," recommend measures for the most affected countries to take and assess Zika's possible association with neurological disorders, the WHO's Aylward said.

"There is a lot of uncertainty about some of the real basics about this disease," Aylward said from Geneva.

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U.S.-based researchers Daniel Lucey and Lawrence Gostin had called for just such a meeting this week in the Journal of the American Medical Association, criticizing the WHO for not stepping up sooner.

"The very process of convening the committee would catalyze international attention, funding, and research," Lucey and Gostin wrote in an article published Wednesday. "While Brazil, PAHO and the CDC have acted rapidly, WHO headquarters has thus far not been proactive, given potentially serious ramifications."

After Chan's announcement about next week's meeting, Gostin urged the WHO leader to "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," said the Georgetown University public health expert, "to be overprepared than to wait until a Zika epidemic spins out of control."

With no vaccine, controlling mosquitoes is key

No medicines are available to treat those with the Zika virus, and there are no vaccines to prevent it. But Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said Thursday that clinical trials on a vaccine could begin this year.

The lack of treatment and preventive options has led officials in Colombia, Jamaica and El Salvador to advise women to avoid getting pregnant so long as the Zika threat remains. Eduardo Espinoza, El Salvador's vice minister of health, recommended that women should "plan their pregnancies and try to avoid getting pregnant this year and the next."

The WHO isn't going that far, nor does it plan to anytime soon, according to Aylward. The agency will more likely focus on advising women in the Americas who want to get pregnant to reduce their risk of mosquito bites. The United States is urging pregnant women not to fly to countries battling the virus.

Authorities are also focusing on containing the Aedes mosquito species, which spreads the disease. These mosquitoes have spread flaviviruses such as Zika before -- for example, dengue fever, in South America, Central America and as far north as Florida and Texas.

And the regularly occurring global weather phenomenon known as El Niño is expected to make things worse by increasing mosquito populations, the WHO's Chan said.

Keeping down the number of what Schuchat called "aggressive daytime biters" is tough. Microbiologist Brian Foy pointed out that Aedes mosquitoes "can replicate in flower vases and other tiny sources of water."

"Community mosquito control may be difficult," Schuchat said. "The current methods that we have may have shortcomings."

Canada requests certain blood donors wait

Canadian Blood Services, which manages most of Canada's supply of blood and blood products, is asking all potential donors who have traveled anywhere other than Canada, the United States or Europe to delay donating blood until one month after their return.

The agency said it is working with Health Canada and Héma-Québec to establish the length of time for delaying donations, but for now the recommendation is one month.

"The risk of Zika virus transmission from a Canadian donor to a blood recipient is very low. This deferral period is being introduced as a precaution, since to date, there has been no evidence of Zika virus transmission by transfusion causing illness in a recipient," the agency said in a statement.

The American Red Cross, the largest blood collection organization in the United States, said it is closely monitoring the virus but isn't taking a similar precaution, at least not yet.

"We are evaluating, as part of the AABB Transfusion Transmissible Disease committee, whether to ask donors to self-defer for 28 days following their return to the U.S. if they traveled to areas with ongoing Zika outbreaks," said Dr. Susan Stramer, vice president of scientific affairs at the American Red Cross.

She said that the Red Cross will continue to use safety measures to protect the blood supply from Zika virus as well as other mosquito-borne viruses.

Schuchat said the FDA is looking into blood supply issues for donors and travelers. The FDA regulates blood collection and manufacturing of blood products in the United States.