By Donald G. McNeil Jr., Simon Romero and Sabrina Tavernise

New York Times News Service

Something strange was happening in August in the maternity wards of Recife, a seaside city perched on Brazil’s easternmost tip, where the country juts into the Atlantic.

“Doctors, pediatricians, neurologists, they started finding this thing we never had seen,” said Dr. Celina Turchi, an infectious diseases researcher at the Oswaldo Cruz Foundation, a prominent scientific institute in Brazil.

“Children with normal faces up to the eyebrows, and then you have no foreheads and very strange heads,” she recalled, referring to the condition known as microcephaly. “The doctors were saying, ‘Well, I saw four today,’ and, ‘Oh that’s strange, because I saw two.’”

Aside from their alarming appearance, many of the babies seemed healthy.

“They cried,” Turchi said. “They breast-fed well. They just didn’t seem to be ill.”

Doctors were stumped.

They did not know it then, but they were seeing the first swell of a horrifying wave. A little-known pathogen — the Zika virus, carried by mosquitoes — had been circulating in Brazil for at least a year. It would later become the chief suspect in the hunt to work out what had happened to those newborns.

Since then, those tiny babies have led the World Health Organization to declare a public health emergency. They have prompted warnings to pregnant women to avoid countries where the virus is circulating, even to refrain from unprotected sex with men who have visited those countries, following a report of sexual transmission of the virus in Dallas last week.

They have led health ministers of five countries to say something so unthinkable that none had ever uttered it before: Women, please delay having children.

The children stricken with microcephaly, or abnormally small heads, have doctors everywhere asking: What is this virus? How could it have been around for almost 70 years without us realizing its power? What do we tell our patients about a bug that can hide in a mosquito’s proboscis and a man’s semen, even in human saliva or urine? What do we tell young women who ask if their unborn babies are safe?

“This epidemic is an unfolding story,” said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. “As with Ebola, this virus is something that could exist for years under the radar, and we don’t know until we get thousands of cases what it really does.”

To doctors in Recife, whatever was striking the babies seemed to have fallen like a bolt from the blue. In reality, it had been building for months.

A year earlier, doctors say, the first patients had started trickling into public hospitals in Natal, capital of the state of Rio Grande do Norte. Many patients lived on the city’s margins, others in settlements dotted across the sertão, northeast Brazil’s arid hinterland.

Almost all had the same symptoms: a flat pinkish rash, bloodshot eyes, fever, joint pain and headaches. None were desperately ill, but the similarities were striking.

“That scared some patients and doctors, and my team,” said Aline Bezerra, a nurse and the municipal epidemiologist. “We knew nothing other than that it might be some kind of light dengue.”

Tests ruled that out, along with other common viruses, but the patients kept coming.

‘Mysterious disease’

By March 2015, the spread of a “doença misteriosa” — the mystery disease — had become impossible to ignore. It appeared in two more states nearby. Then it reached Salvador, a city of 2.5 million.

Doctors speculated that it was an allergy; that it was roseola, a childhood illness; that it was a new variant of Fifth Disease, a facial rash that gives children a “slapped-cheek” look.

“People were claiming it was polluted water,” said Dr. Gúbio Soares, a virologist at the Federal University of Bahia in Salvador. “I began thinking it was something transmitted by mosquitoes.”

Working in his modest lab with a colleague, Dr. Silvia Sardi, Soares kept testing blood samples. Other doctors were doing the same; more than 6,800 samples were tested. Finally, in April 2015, Soares and Sardi were sure: It was Zika.

“I actually felt a sense of relief,” Soares said. “The literature said it was much less aggressive than viruses we already deal with in Brazil.”

In the capital, Brasília, the health minister at the time, Dr. Arthur Chioro, felt the same way.

“Zika virus doesn’t worry us,” he told reporters in May. “It’s a benign disease.” Dengue hemorrhagic fever, on the other hand, killed hundreds of Brazilians each year.

But on ProMED Mail, an online service run by the International Society for Infectious Diseases, the reaction was not so sanguine.

“The arrival of Zika virus in Brazil is not good news,” wrote Thomas Yuill, an emeritus professor of veterinary science and wildlife ecology at the University of Wisconsin-Madison.

Not only did Brazil have “abundant mosquitoes and a large population of susceptible people,” he wrote, but so did much of the Americas.

Zika in the U.S.

The first case of Zika infection detected in New York City was found in December 2013 — six months before the virus is thought to have reached Brazil — in a 48-year-old traveler who has asked to remain unidentified.

When he walked into Traveler’s Medical Service on Madison Avenue, he had just returned from a long trek through Ecuador, Peru, Bolivia, Chile, Easter Island and Hawaii, with a stopover in French Polynesia. Dyan Summers, the nurse who first saw him, said he had a pinkish rash.

“I took one look and said, ‘Dengue fever,’” she recalled in an interview last week. “He said, ‘I’m not so sure. I think it’s Zika.’”

Summers was startled: “I’d heard of Zika, but nobody was thinking about Zika.”

In May, after it was confirmed that Zika was circulating in Brazil, it took only a few weeks for doctors to suspect that Chioro, the health minister, had been mistaken. There were hints that the virus was anything but benign.

In Maceió, Recife and other cities, cases of Guillain-Barré began to spike. Dr. María Lúcia Brito, a neurologist in Recife, saw 50 patients with it in 2015, up from 14 the year before.

“It was obvious — a shift occurred when Zika cases started to rise,” she said.

Microcephaly connection

In early October, the national health ministry asked Turchi to investigate. Zika’s connection to microcephaly was suspected but very difficult to confirm. Turchi set up a quick “case control” study, the epidemiologist’s classic tool, comparing babies born with the condition and those without it.

Dr. David Heymann, chairman of the World Health Organization committee that recommended the declaration of the public health emergency, said in an interview last week that very tool was what his committee needed to prove whether Zika causes microcephaly.

“Sorting out a rare event will take a lot of women,” he said, and they must be followed for months.

Turchi did not have months. She called every scientist she knew, and they came from all over Brazil. One flew in from London. Turchi gave the group a name: MERG, the Microcephaly Epidemic Research Group.

A turning point came in early November: Dr. Adriana Melo in Paraíba State, just north of Recife, had drawn amniotic fluid from a pregnant woman and found Zika virus in it. Then brain tissue from two stillbirths was tested. Again, Zika.

“At last we had a road to follow,” Turchi said. “A map.”

Loose on a continent where no one is immune, Zika has the potential to infect tens of millions of people. It is now being transmitted in 33 countries with about 600 million inhabitants, the WHO says. Health officials in Brazil are investigating thousands of reported cases of microcephaly that may be linked to the virus.

Now that the world is alert to the danger and is fighting back, and women are even contemplating delaying pregnancies, scientists say it is unlikely that Brazil’s national nightmare will be repeated elsewhere on such a scale.

“I’m more comfortable now,” Turchi said. “I see so many people working as a team and so much international concern. Now it has become clear to the whole world.”