Zika, first identified more than 50 years ago, has alarmed public health officials in recent months because of its possible association with thousands of suspected cases of brain damage in babies. The WHO has estimated that the virus will reach most of the hemisphere and infect up to 4 million people by year's end.

WHO Director-General Margaret Chan said at a media briefing Monday that the primary reason for the designation was the "strongly suspected" causal relationship between Zika and the rare congenital condition called microcephaly. Even before that association is scientifically confirmed or disproved, members of an 18-member advisory panel said the seriousness of the cases being reported required action. Chan concurred, saying the consequences of waiting were too great.

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“Even the clusters of microcephaly alone are enough to declare a public health emergency because of its heavy burden" on women, families and the community, Chan said.

In addition, she noted, there is significant concern given the lack of vaccines and rapid, reliable diagnostic tests, as well as the absence of any viral immunity in the population since the Americas are being affected for the first time. A potential link to Zika is also suspected in adults who have been diagnosed with Guillain-Barré syndrome, a rare condition that can cause paralysis.

According to the latest figures on the epicenter of the outbreak, Brazil has 4,000 suspected cases of microcephaly, with 270 confirmed with evidence of an infection.

The WHO declaration represents its highest level of alert and is only invoked in response to the most dire threats. The first time was in 2009 during the H1N1 influenza epidemic that is believed to have infected up to 200 million worldwide; the second in May 2014 when a paralyzing form of polio re-emerged in Pakistan and Syria; and the third in August 2014 with Ebola in West Africa.

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Identifying a global public health emergency now allows the WHO to help coordinate surveillance efforts, such as recording and monitoring cases of Zika, microcephaly and other neurological complications. Officials said countries need to standardize their surveillance for microcephaly, in which an abnormally small head signals incomplete brain development.

Zika’s emergence as a major concern has taken infectious disease experts by surprise. The virus has been popping up in various parts of the world for decades, but individuals have typically only suffered mild symptoms such as a rash or body aches. Nearly all those infected in the past recovered fully, though there were rare cases of complications.

In recent months, however, as reports about brain-damaged newborns began increasing, health officials have worried about Zika's impact on fetal development and have focused on trying to protect pregnant women from the virus. In January, the U.S. Centers for Disease Control and Prevention issued a travel advisory urging pregnant women to avoid travel to areas where the virus is actively spreading. That advisory has been expanded repeatedly and now lists more than two dozen countries and territories in Central America, South America and the Caribbean.

The WHO said Monday there is no reason for travel or trade restrictions at this time. Instead, health officials there reiterated their advice to pregnant women to wear appropriate clothing, use insect repellent and take other practical measures, such as sleeping under a bed net to avoid being bitten.

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Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said the WHO’s declaration is the "official global sounding bell that governments and others need to start really paying attention to this."

Infectious disease experts and others have pressured the WHO to escalate its response to Zika for several months, warning of the mistakes world leaders made during the Ebola crisis when a lack of coordination delayed quarantines and treatment. Chan seemed to reflect their calls when she asked rhetorically during Monday's media briefing, "Can you imagine if we do not do all these works now and wait until the science comes out, then people will say, 'Why don't you take action?'"

Bruce Aylward, the WHO's director of outbreaks and health emergencies, said that the evidence pointing to both a “temporal and geographic association” between Zika and microcephaly was strong.

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"This is definitely the right measure to be taking at this time based on the information available," he said.

Some public health experts said the WHO did not go far enough.

Lawrence Gostin, a public health and law expert at Georgetown University, said it was a mistake that WHO did not issue a travel alert for pregnant women visiting Zika-affected countries. The organization's failure to do so puts it at odds with the CDC's travel warning to pregnant women.

The countries affected typically have younger populations, which means more women of childbearing age who will be worrying about the virus and its potential harm. Some of these countries have had among the world’s higher birth rates recently.

From 2011-2015, according to data from the World Bank, Guatemala had 27 births per 1,000 people; Haiti, 25; and Bolivia, 24. The rate in Brazil – South America’s most populous country by far – was 15 births per 1,000 people during that same period.

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Gostin said parents concerned for the welfare of their daughters should advise them to avoid going to affected areas if they are pregnant. "If this wave of Zika infections is followed by a wave of birth defects in nine months, it would be unconscionable," he said in a statement.

Some countries are asking women to indefinitely delay pregnancy. But most Latin American countries also restrict access to contraception and abortion, and that puts a heavy and unfair burden on young and often poor women, he said.

Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Texas, sees most countries in the Americas as ill-equipped to preemptively fight the disease. It already has moved rapidly from South America north into Mexico.

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“What do you do that’s meaningful to prevent microcephaly?” Hotez asked. “It’s going to go beyond the current health budget of these countries. You’re going to have to get the buy-in of the presidents of these nations.”

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Zika may even be harder to defeat than Ebola, according to Hotez. While that virus was more deadly and could spread from human to human, it was easier to contain by isolating infected patients and ensuring safe burials. Ebola affected roughly 30,000 people in four West African countries. Zika could affect millions of people throughout the entire Western Hemisphere.

“This is more challenging and more complicated because of the vector and what you’re up against,” Hotez noted.

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In an interview with the Reuters news agency, Brazilian Heath Minister Marcelo Castro said the outbreak is his country is worse than previously believed because an estimated 80 percent of people who become infected with the virus do not exhibit known symptoms.

Castro also said every municipality in Brazil will be required to report all Zika cases to a central database starting next week. In further controls, Brazil will join other nations in banning blood donations from people who had the virus.

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Castro warned last week that Brazil was “badly losing” the battle against the mosquito blamed for spreading Zika and said that more than 220,000 members of Brazil’s military would be mobilized in eradication efforts. The plans included distributing mosquito repellent to about 400,000 pregnant women, according to Brazil’s O Globo newspaper.

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Brian Murphy contributed to this report.

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