Gleyse Kelly da Silva holds her daughter Maria Giovanna as she sleeps in their house in Recife, Pernambuco state, Brazil, on Wednesday, Jan. 27, 2016. Brazilian officials still say they believe there's a sharp increase in cases of microcephaly and strongly suspect the Zika virus, which first appeared in the country last year, is to blame. The concern is strong enough that the U.S. Centers for Disease Control and Prevention this month warned pregnant women to reconsider visits to areas where Zika is present.

GENEVA — The World Health Organization has declared a global emergency over the explosive spread of the Zika virus, which has been linked to birth defects in the Americas, calling it an “extraordinary event” that poses a public health threat to other parts of the world.

The U.N. agency took the rare step despite a lack of definitive evidence proving the mosquito-borne virus is causing a surge in babies born with brain defects and abnormally small heads in Brazil and following a 2013-14 outbreak in French Polynesia.

Monday’s emergency meeting of independent experts was called in response to the spike in babies born with microcephaly in Brazil since the virus was first found there last year. Officials in French Polynesia also documented a connection between Zika and neurological complications when the virus was spreading there two years ago, at the same time as dengue fever.

“After a review of the evidence, the committee advised that the clusters of microcephaly and other neurological complications constitute an extraordinary event and public health threat to other parts of the world,” WHO Director-General Dr. Margaret Chan said.

WHO, which was widely criticized for its sluggish response to the 2014 Ebola crisis in West Africa, has been eager to show its responsiveness this time. Despite dire warnings that Ebola was out of control in mid-2014, WHO didn’t declare an emergency until months later, after nearly 1,000 people had died.

“If indeed, the scientific linkage between Zika and microcephaly is established, can you imagine if we do not do all this work now and wait until the scientific evidence comes out?” Chan said. “Then people will say, ‘Why didn’t you take action?'”

WHO estimates there could be up to 4 million cases of Zika in the Americas in the next year, but no recommendations were made to restrict travel or trade.

“It is important to understand, there are several measures pregnant women can take,” Chan said. “If you can delay travel and it does not affect your other family commitments, it is something to consider.”

“If they need to travel, they can get advice from their physician and take personal protective measures, like wearing long sleeves and shirts and pants and using mosquito repellent.”

Last month, the U.S. Centers for Disease Control advised pregnant women to postpone visits to Brazil and other countries in the region with Zika outbreaks. But officials said it was unlikely the virus could cause widespread problems in the U.S.

The last such public health emergency was declared for the devastating Ebola outbreak in West Africa, which killed more than 11,000 people. Similar declarations were made for polio in 2013 and the 2009 swine flu pandemic.

Such emergency declarations are meant as an international SOS signal and usually trigger increased money and efforts to halt the outbreak, as well as prompting research into possible treatments and vaccines. There are currently no licensed treatments or vaccines for Zika.

WHO officials say it could be six to nine months before science proves or disproves any connection between the virus and babies born in Brazil or elsewhere with abnormally small heads.

Zika was first identified in 1947 in Uganda but until last year, it wasn’t believed to cause any serious effects; about 80 percent of infected people never experience symptoms. The virus has also been linked to Guillain-Barre syndrome, which causes muscle weakness and nerve problems.

Michael Osterholm, an infectious diseases expert at the University of Minnesota, said it was still unclear how Zika had evolved since it first emerged in Africa, but that even minor genetic changes might have major consequences.

“It could have just been some point mutation (in the virus) that has now made a big difference,” Osterholm said, adding it would likely take years to curb the mosquito populations capable of spreading Zika — and before local populations gain enough immunity for the number of cases to fall.

Jimmy Whitworth, an infectious diseases expert at the London School of Hygiene and Tropical Medicine, said we might soon see babies born elsewhere with malformed heads as the virus becomes entrenched in other countries.

“It could be that we’re getting the strongest signal in Brazil,” he said. “But having these cases occurring and pinning it to Zika is tough.”

Whitworth said it was important for WHO to act quickly, despite the lack of definitive evidence that Zika is responsible for the surge in microcephaly cases.

“For situations like this, you have to essentially have a ‘no regrets’ policy,” he said. “Maybe this will be a false alarm when more information is available months later, but it’s serious enough on the evidence we have right now that we have to act.”

This 2006 photo provided by the Centers for Disease Control and Prevention shows a female Aedes aegypti mosquito in the process of acquiring a blood meal from a human host.

Aedes mosquitoes

Behind the tropical disease outbreaks that have exploded in Latin America is a tiny menace that just won’t go away.

It’s called the Aedes aegypti, a species of mosquito that has played a villainous role in public health history and defeated attempts to eradicate it from the New World.

Five things to know:

First in Africa

Aedes aegypti is a small, dark, hot-weather mosquito with white markings and banded legs. Scientists believe the species originated in Africa, but came to the Americas on slave ships. It’s continued to spread through shipping and airplanes. Now it’s found through much of the world, including the southern United States.

Once yellow fever

Early in the 20th century, scientists showed it was the engine behind devastating yellow fever outbreaks. It became known as the yellow fever mosquito, although that name has been used less often since a successful vaccine was developed against yellow fever. Since then, it’s also been identified as a carrier for other tropical illnesses such as dengue fever, chikungunya and Zika fever. Scientists are investigating whether other types of mosquitoes are spreading Zika in Latin America and the Caribbean, too.

City dweller

Aedes aegypti is the primary spreader of Zika and some other tropical diseases, largely because of its unusually cozy relationship with people. While other species thrive in more rural areas, or at least in parks and gardens, this is a domesticated species — sort of a housecat mosquito — accustomed to living in apartment buildings and city centers. It prefers biting people to animals and likes to feed indoors, during daylight hours. Also, it is a hardy bug that can be particularly challenging to get rid of.

Killed off, it came back

In the early 20th century, many countries developed programs to destroy all Aedes aegypti mosquitoes through spraying chemicals and other measures. By 1970, it was eradicated from much of South America — including Brazil. But many mosquito-control programs were cut back. Reasons included budget cuts, and concerns about the use of some insecticides, and the yellow fever vaccine made the mosquito’s elimination less critical.

The species roared back. It played a key role in the reemergence of dengue fever in Latin America and the Caribbean, and the recent spread of chikungunya and Zika virus.

Going for blood

Female mosquitoes drink human blood for nutrients used in making eggs. After a female bites an infected person, it can spread the virus through its saliva to its next human victim. Some scientists think Zika may have arrived in Brazil in 2014, carried by visitors attending World Cup soccer games. Perhaps one or more infected visitors were bitten by mosquitoes and started the chain reaction, said Jeffrey Powell, a Yale University mosquito expert who works in Brazil.