Everything you ever wanted to know about the Zika virus and its spread across North and South America. (Daron Taylor,Claritza Jimenez/The Washington Post)

Everything you ever wanted to know about the Zika virus and its spread across North and South America. (Daron Taylor,Claritza Jimenez/The Washington Post)

Global health authorities and government officials are mobilizing to battle the fast-spreading Zika virus, sending rapid-response teams to affected regions, issuing travel warnings for pregnant women, accelerating vaccine trials and even deploying mosquito-fighting troops to hard-hit areas in Brazil.

Stung by criticism that the world’s response to the Ebola epidemic was halting and disjointed, officials in Latin America, the United States and Europe say they are determined to do better against a new foe that, in some ways, is more daunting than the hemorrhagic virus that killed 11,000 people in West Africa.

But their efforts are being hobbled by a fundamental lack of understanding of a disease that has spread to nearly three dozen countries and is moving so quickly that some experts estimate it could infect as many as 3 million to 4 million people within 12 months.

Unlike the highly lethal Ebola, which had been studied extensively by the time it emerged in Guinea in late 2013, Zika wasn’t considered a menace. Most infected people didn’t even show symptoms. For decades, Zika got little scrutiny before showing up in Brazil in May.

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1 of 65 Full Screen Autoplay Close Skip Ad × How countries are combating the Zika virus View Photos The Zika virus, a mosquito-borne disease suspected of causing serious birth defects, is expected to spread to all countries in the Americas except Canada and Chile, the World Health Organization said. Caption The mosquito-borne disease suspected of causing serious birth defects is expected to spread to all countries in the Americas except Canada and Chile and remains a global health emergency, according to the World Health Organization. Oct. 27, 2016 A health ministry worker fumigates a house to kill mosquitoes during a campaign to prevent Zika in Managua, Nicaragua. Oswaldo Rivas/Reuters Buy Photo Wait 1 second to continue.

Now, for the first time, Zika is suspected of being the culprit behind a constellation of devastating birth defects, most notably microcephaly, a rare condition in which babies are born with head and brain abnormalities. In adults, the virus has been associated with another rare condition, called Guillain-Barré syndrome, that can result in paralysis and even death. But, even at this point, scientists are not able to say definitively that Zika is the cause of the problems.

The mystery surrounding the virus — officials aren’t sure what will turn up next — makes it “much more insidious, cunning and evil” than Ebola, said Bruce Aylward, a World Health Organization official who helped coordinate the organization’s Ebola response and is now leading the Zika effort.

The urgency to understand the virus, along with pressure not to repeat its slow-footed reaction to the devastating Ebola crisis, prompted the WHO to move faster this time to declare a global public health emergency.

“This emergency is because of what’s unknown,” said David Heymann, an infectious-disease professor at the London School of Hygiene and Tropical Medicine and chairman of the expert committee that urged WHO to call a global health emergency. “The Ebola emergency was because of what was known.”

But declaring an emergency is just the beginning, and many public health experts are pressing for quick and aggressive steps. There has been no public announcement yet of a strategic plan with a timetable and cost estimate to make it work, said Lawrence Gostin, a global health law professor at Georgetown University.

“Calling an emergency only scratches the surface,” he said. “There has to be action that follows it.”

On Saturday, the WHO said it plans to soon release an “overview of needs and requirements” to fight the epidemic.

‘We need to act now’

As the WHO works to detail the threat to affected countries, some government officials and others are planning for worst-case scenarios. Scientists around the world are scrambling to learn why so many people are getting sick and how those illnesses are related to the virus, particularly for pregnant women and others at greatest risk. So far, only Brazil has reported a surge in cases of microcephaly. French Polynesia, Brazil, El Salvador, the French territory of Martinique, Colombia, Suriname, Venezuela and Honduras have all reported increases in people with Guillain-Barré syndrome, according to the WHO.

[Zika tied to major eye defects in babies with microcephaly, study finds]

Some involved in the Zika response say the international coordination is already much better than it was during the Ebola crisis. Part of that has less to do with the institutions involved than with the people who work for them, many of whom formed tight bonds during the last epidemic.

“Groups are just linked up in a way that wasn’t true in the first nine or 10 months of Ebola,” said Jeremy Farrar, director of the Wellcome Trust, a London-based biomedical research charity that worked in West Africa. “There’s much more free flowing of information about what each other is doing. I’m not saying it’s perfect, but it is completely different.”

Farrar said the WHO and others need to take advantage of the momentum and move decisively. “The worst situation is we under-react for a year and an epidemic gets out of control and causes death on the scale that Ebola did,” he said. “If we’ve taken a lesson from Ebola, it is the need to be bolder and more courageous. To say, ‘We need to act now.’ ”

But it is not always clear what that decisive action should be, or how to marshal the political and financial resources needed to make it happen.

One challenge involves the way people are infected. Ebola requires close human contact for person-to-person transmission, so health officials knew essentially what needed to be done, even if help was slow in coming: warn people not to touch the dead bodies of loved ones, build centers to treat and isolate patients, and shore up fragile health systems.

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But because Zika is spread primarily by the Aedes aegypti mosquito, curbing the epidemic is more complicated. Much depends on individual governments — and citizens — mounting aggressive efforts to kill mosquitoes and deprive them of breeding grounds.

Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine, said it’s worth considering a “military-style campaign,” such as the effort in the middle of the past century in which authorities eradicated Aedes aegypti in much of South America and parts of the Caribbean in a bid to stop dengue fever. But he knows that won’t be fast or easy.

“It’s labor intensive,” Hotez said. “There would have to be a lot of political will.”

The Centers for Disease Control and Prevention, one of the lead agencies in the fight against Ebola, is sending teams to affected regions to research Zika and help with mosquito control. In its Atlanta-based emergency operations center, which is on the highest level of activation, hangs an enormous photo of the Aedes aegypti.

This mosquito, considered one of the most effective disease-spreading vectors in the world, bites only humans and attacks aggressively during the daytime, often biting four to five people at one blood meal. Its bite is relatively painless, so people don’t swat and kill it. It thrives in dense, urban environments, breeding in the tiniest of places, such as a drop of water in a bottle cap. Its larvae don’t necessarily need water to survive, laying dormant during a drought and hatching during the rainy season. All of that makes it a formidable foe.

“It takes near-perfection to stop Ebola, but if you do the right things, you can halt it,” said Thomas Frieden, director of the CDC. “Mosquito control requires meticulous attention to detail, trained staff and, like other parts of public health, it has to be done day in and day out.”

[How climate change could help the spread of Zika and other infectious diseases]

The harrowing road ahead

Throughout the affected region, countries are coming up with different strategies to try to contain the epidemic. While Brazil has mobilized a national effort, deploying soldiers to join health workers, El Salvador is urging women to wait until 2018 to become pregnant. Colombia is concentrating its resources and attention in a few key states.

“This is classic ‘building your boat as you sail it,’ ” said Aylward, the WHO’s man in charge of fighting epidemics. “People said that about Ebola, and that was trying to get a bigger sail on the boat. Here we’re still stitching the sail, and we’re not quite sure what kind of sails you really need.”

An overarching goal is protecting pregnant women. After a Dallas resident was infected by having sex with a person who had contracted the disease in Venezuela, the CDC within days issued new guidelines detailing measures for pregnant women to protect against sexual transmission of Zika. It was only the second reported case of sexual transmission.

Diagnosing Zika in pregnant women also is challenging. The virus stays in the blood only for about a week. After that, it’s difficult to know if a woman was infected because the tests used aren’t sensitive enough to differentiate between Zika and two related viruses on a routine basis. Those other viruses, dengue and chikungunya, are transmitted by the same type of mosquito, and many of the Zika-affected countries continue to battle outbreaks of those viruses.

The CDC has sent out material to state labs and health departments to perform 30,000 tests for Zika. It’s getting ready to send out material for tens of thousands more tests. But that’s not going to be enough to promptly test all the pregnant women who will be returning from Zika-affected countries, Frieden said during recent congressional testimony.

In any given year, 30 million to 40 million people travel back and forth between the United States and South America and the Caribbean, including half a million pregnant women.

[Why the United States is so vulnerable to the alarming spread of Zika virus]

Meanwhile, affected countries are struggling to put in place systems to detect signs of microcephaly and Guillain-Barré syndrome, and to brace for the far-reaching effect on families trying to care for infants who might have disabilities. In poor countries, that will be especially difficult.

“These women will need care that may be beyond what we can provide in a place like Haiti,” said Joia Mukherjee, chief medical officer for Partners in Health, a Boston-based nonprofit that provides health care in Haiti. There aren’t enough ultrasound machines to screen pregnant women, or specialists to provide long-term support for disabled children.

“This is going to be a huge burden to a poor family,” she said.

As health experts grapple with the latest epidemic, some say that the biggest lesson from Ebola — the importance of building robust health systems so countries are prepared for newly emerging diseases — has not yet been learned.

“We can’t keep being caught off guard and unprepared,” said Georgetown’s Gostin. “There are going to be global health emergencies, and we need to have funding already mobilized, planning already done. . . . None of that is happening.”