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The spread of Zika virus across the Americas and its suspected link to birth defects is a public health emergency of international concern and deserves urgent attention, the World Health Organization said Monday.

"I am now declaring that a recent cluster of microcephaly and other neurological abnormalities reported in Latin America following a similar cluster in French Polynesia in 2014 constitute a public health emergency of international concern," WHO director general Dr. Margaret Chan told a news conference.

WHO said last week that Zika was spreading “explosively” across the Americas and predicted 3-4 million people could be infected within a year.

It would not have been of concern –Zika normally causes only mild symptoms at worst – but Brazil noted a marked increase in cases of a severe and devastating birth defect called microcephaly that coincided with Zika’s arrival. Some doctors also fear the virus may cause a paralyzing condition called Guillan Barre syndrome.

"It’s only because of this association, if it is proven, that Zika could be considered as a public health emergency of international concern."

Some health experts have accused WHO of acting too slowly and the organization’s been under pressure to move more quickly against Zika.

Chan said travel or trade restrictions are not called for at this time. The most important measures will be to protect people from the mosquitoes whose bites transmit the virus. She said pregnant women who can delay travel to affected regions should; the U.S. Centers for Disease Control and Prevention gave similar advice last month.

"A coordinated international response is needed to improve surveillance, the detection of infections, congenital malformations, and neurological complications, to intensify the control of mosquito populations, and to expedite the development of diagnostic tests and vaccines to protect people at risk, especially during pregnancy," Chan said.

She made the decision after a meeting of WHO's 18-member emergency committee.

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The spread of Zika alone would not be an emergency, said Dr. David Heymann, Chair of WHO's emergency committee. "Zika as we understand today is not a clinically significant infection," Heymann said. "It’s only because of this association, if it is proven, that Zika could be considered as a public health emergency of international concern. That’s why it was a very difficult deliberation."

WHO makes clear that it is not certain that Zika causes microcephaly and says a lot more work needs to be done to show it. However, that work needs to be done quickly.

Babies with microcephaly have unusually small and undeveloped brains. It can cause miscarriages or stillbirths and babies who survive are disabled for life. Viruses such as rubella, malnutrition and alcohol use during pregnancy can all cause it.

Scientists became more concerned about Brazil's reports of an upsurge when autopsies on babies that miscarried with microcephaly showed they were infected with Zika.

“The evidence is growing and it is getting strong," Chan said.

"If we do not do all this work now and wait until the scientific evidence comes out, people will ask why we did not take action?" Chan added.

"Without a clear strategy and ample resources, sounding an alert is simply not enough.”

"I don’t think people are concerned about raising false concern. I think they are concerned about getting to the bottom of what’s causing microcephaly," Heymann said.

Related: How Worried Should You be About Zika?

There is not a whole lot WHO can do. The organization doesn’t have a lot of cash to pour into research or immediate medical care. But the largely bureaucratic declaration can encourage countries to donate money, to coordinate efforts and, of course, it raises the profile of a disease outbreak.

"What matters now is for WHO to take decisive action. Margaret Chan was silent on what strategy the organization will take on the ground to control Zika and how she will mobilize the major funding needed for surveillance, mosquito control, and critical research," said Lawrence Gostin of Georgetown University, who had been calling for WHO to act.

"Without a clear strategy and ample resources, sounding an alert is simply not enough.”

Because human pregnancy lasts for so long, the true consequences of a Zika outbreak might not be immediately clear, Gostin and other experts have argued.

“If this wave of Zika infections is followed by a wave of birth defects in nine months, it would be unconscionable,"Gostin said.

"Some countries are asking women to indefinitely delay pregnancy, but most Latin American countries also restrict access to safe contraception and abortion. It is unfair to put such a heavy burden on young, often poor, women. WHO should stand up for women’s health and reproductive rights.”

The idea of a public health emergency of international concern has only existed since 2007. WHO has declared the emergencies for the H1N1 swine flu pandemic in 2009; for a resurgence in polio and for the Ebola epidemic in West Africa.

WHO has declined to call the spread of the Middle East Respiratory Syndrome (MERS) a public health emergency of international concern.