On a Saturday morning in April of 2014, Nenad Macesic, a thirty-one-year-old doctor-in-training, received an urgent phone call from the emergency room of Austin Hospital, just outside Melbourne, Australia. Lean and taut, with a swirl of dark hair, Macesic resembles an aspiring urban d.j. In fact, by night he spun electronica in clubs around Melbourne; by day he was a fellow in infectious diseases. The call concerned a woman in her late forties who had come to the hospital complaining of a fever, headaches, and an unusual rash.

Travel-related illnesses may be an Australian obsession: foreign contagions brought into the country can spread like, well, rabbits. The woman in the E.R. had just returned from the Cook Islands, an isolated spray of atolls in the South Pacific, where she and her husband had been attending a family funeral. Other people at the funeral had been sick with mysterious fevers, but she hadn’t made much of it. Now that she was home, though, a mild headache had progressed to a full, persistent throb. Migratory pains appeared in her joints, and an angry, blanching rash—the kind that pales when you press it—was now blooming across her torso.

When Macesic entered her hospital room, the woman, a textile worker, looked more medically stable than he had expected she would. She spoke in measured sentences, with no sign of confusion or delirium. But Macesic was struck by her strange rash—vivid raised red dots coalescing into islands—and the color of her eyes (pink, with streaks of vermillion), which was indicative of conjunctivitis, a symptom of certain viral infections.

Was it dengue? Macesic wondered. Dengue—colloquially known as breakbone fever, because of the intense corkscrews of pain that can occur in the bones, muscles, and joints—is caused by a mosquito-borne virus, and was endemic in the Cook Islands. But the woman’s symptoms seemed too mild for dengue: the disease can cause catastrophic drops in white blood cells and platelets, but her blood counts were nearly normal. Could it be chikungunya? Another mosquito-transmitted viral fever, chikungunya can leave its victims with months, or even years, of wracking joint pains. But this woman’s joint pains and swellings weren’t severe. It was as if she had acquired a milder variant of those diseases—a more temperate cousin. And the conjunctivitis was a tipoff: neither chikungunya nor dengue is usually accompanied by those blood-tinged eyes.

Macesic decided to consult an online reporting system called ProMED, which tracks infectious diseases around the world. Even surfing the site casually takes a fair amount of fortitude: one day this month, there were eleven new reports on the site, including an undiagnosed measles-like disease that killed forty children in rural Myanmar; anthrax outbreaks among deer in Siberia; food poisoning from cyclospora at a Mexican resort; and a form of strep, normally found in horses, that sickened a woman in Washington State and killed her mother.

As Macesic went through previous entries in ProMED’s database—malaria in Oman, Lassa fever in Nigeria—he found a cluster of cases in French Polynesia, some six hundred miles east of the Cook Islands, that seemed remarkably similar to the woman’s condition: a dengue-like, mosquito-borne viral syndrome, but with a milder course. Those cases had been attributed to a little-known virus called Zika, a member of a family of RNA viruses that includes dengue, West Nile, and yellow fever. (Zika gets its name from the Ugandan forest where the virus was first found, in a monkey, in the nineteen-forties.) Macesic sent the woman’s blood to a specialized laboratory for viral analysis.

The next morning, the woman’s husband arrived at the hospital, enveloped in the same diffuse, blanching rash. By the end of the week, the woman’s blood test had come back positive for the Zika virus. The husband, however, had no detectable virus in his blood: he had seemingly cleared the infection almost completely. In both cases, Macesic noted, the symptoms had also begun to resolve on their own. He figured that the man and the woman had been bitten by Zika-carrying mosquitoes. (The sexual transmission of Zika had been described in one prior case report, but Macesic did not know about it.) Macesic wrote the case up as an abstruse curiosity—a medical “quiz”—for an infectious-diseases journal. “The illness is typically mild and self-limited, with resolution over 1 week,” he noted. “In a previous outbreak with 49 confirmed cases of ZIKV, no deaths, hospitalizations, or hemorrhagic complications were reported, but neurological complications . . . have been described.”

Medical students are often taught a piece of diagnostic wisdom: “When you hear hoofbeats, think horses, not zebras.” But this case, a rare illness that closely resembled common ones, was a classic zebra. Macesic didn’t expect to encounter it again—at least, not anytime soon.

It was On March 2, 2015, less than a year after Macesic had seen the two Zika cases from the Cook Islands, that health authorities in Brazil notified the World Health Organization about a viral illness, marked by mild fevers and skin rashes, that was moving swiftly through its northeastern states. By the end of April, nearly seven thousand cases had been reported. Health officials eventually determined that the illness was Zika. One theory, among many, for the virus’s appearance in Brazil is that it arrived in 2013, when Tahiti’s soccer team, and hordes of fans, descended upon the country for the Confederations Cup. Zika travelled to Brazil, then, as viruses prefer to travel these days—on transcontinental airplanes.

In mid-July, 2015, there was more disturbing news. Forty-nine cases of Guillain-Barré syndrome—a neurological condition, marked by flaccid paralysis, that can be associated with an aberrant immune response to a virus—were reported in Brazil, echoing a sharp increase in the syndrome which was noticed in Polynesia during the Zika outbreak there. Zika had also begun to move through Cape Verde and Colombia. Macesic recalled tracking it on ProMED—“following Zika around the globe had become my small addiction,” he told me. “But the most devastating complication, the one that virtually no one had really anticipated, was still to come.”

In the late summer, doctors in Brazil noted an unusually large number of babies born with microcephaly. Such babies have smaller heads and shortened foreheads, a result of the inadequate growth of parts of the fetal brain; they can suffer cognitive dysfunction, seizures, developmental delays, and problems with hearing and eyesight. In early November, Brazilian health officials reported a hundred and forty-one suspected cases of microcephaly. By late January, the number of reported cases skyrocketed to nearly four thousand. Alarmed by this sudden rise—in previous years, the nationwide annual incidence had been estimated at fewer than two hundred cases—epidemiologists began to investigate. Scouring through case reports and histories, they converged on a prime candidate: Zika infection during early pregnancy. In some cases, scientists suspect, the virus crosses the placenta, infects the developing brain, and kills nerve progenitors. For Zika-infected pregnant women, estimates of the risk of birth defects range widely, from one per cent to thirty per cent.

“We still don’t understand the factors that contributed to the striking number of congenital birth defects seen during this pandemic,” Eva Harris, a professor at U.C. Berkeley’s School of Public Health who studies dengue, Zika, and other emerging infections, told me. “Possible explanations include the vast number of people infected—a numbers game. There could be other factors, such as the viral strain, the genetics of the host, environmental exposures, or immune-related factors, such as prior dengue infection.”