By the time the 48-year-old man showed up at a clinic in New York City he had been sick for almost two weeks. A blotchy, red rash still blanketed his torso and his body ached. He had just gotten over a triple-digit fever, intense lower back pain and a painful eye infection. Five weeks earlier he had embarked on a long vacation to South America and Polynesia but during his trip he had felt fine. He had hopscotched from country to country until he capped his stint through French Polynesia with a trip to Mooréa, an island about 16 kilometers northwest of Tahiti. The South Pacific paradise was teeming with some hungry mosquitos. But he wasn’t worried about the bites. He knew he was up on all his travel vaccinations.

Even after a handful of telltale bumps erupted on his skin he was all right for several days. Then, some 12 hours after leaving Mooréa, he was not. First, he started to feel tired and developed an intransigent rash on the back of his neck. The rash appeared right where his camera strap often rubbed, so at first he did not think much about it. “It was like a mosquito bite gone rogue,” he says. But then the rash started to creep downward across his body. His fatigue grew and his temperature spiked. Soon his eyes turned swollen and red and dripped stringy mucous. His lower back hurt, too, and popping painkillers offered little relief. A tough week followed. Some nine days later he felt better but still had a pernicious red rash that had crept across his back, arms and legs. Soon he made an appointment at a clinic.

Once there, his doctors struggled to make a diagnosis. Mosquitoes can carry a raft of diseases like malaria, dengue, West Nile or chikungunya. But none of those diagnoses seemed to be a perfect fit. Although he had aches and pains typical of dengue or chikungunya, his medical team believed it was unlikely he had contracted those ailments based on where he traveled. His lab test results were equally puzzling. Blood tests showed he had antibodies for West Nile virus and dengue. Yet his team could not even trust those findings. The two viruses come from the same family as several other mosquito-borne pathogens so the lab test may have detected the antibodies – possible holdovers from a yellow fever vaccination or earlier infections – and then falsely indicated he had those maladies instead of one of their viral cousins. That phenomenon, called cross-reactivity, could allow the real disease to fly under the radar, his team said.

Without any definitive, immediate answers he was advised to drink plenty of fluids to replenish liquids he lost through sweat and to take over-the-counter pain medications like Tylenol, as necessary.  Soon the rash cleared up. But a month after the man started to feel ill another round of blood tests revealed something new. His levels of antibodies against dengue and West Nile were still elevated. But the amount of antibodies against a rare tropical disease called Zika had increased fivefold between his first clinic visit and his follow-up tests. The spike in Zika antibodies confirmed what both the patient and the clinic workers suspected: He had the dubious honor of being the first American tourist with a documented case of Zika. His experience was described in the Journal of Travel Medicine earlier this year.

Now, the U.S. Centers for Disease Control and Prevention are steeling themselves for many more Zika cases. The disease is generally pretty mild—on par with flu—but health workers have recently found that a small number of patients seem to go on to develop an autoimmune disorder that can cause nerve damage and paralysis called Guillain–Barré syndrome. “This is a pretty troubling finding,” says Scott Weaver, an expert on mosquito-borne viral diseases at The University of Texas Medical Branch at Galveston. Exactly how many Zika patients have that extreme reaction remains unknown because doctors only linked the two maladies in the past couple years. And because Zika is so often missed—thanks to lab complications or patients’ choice not to seek care—it is challenging to prepare for the possibility of Guillain–Barré syndrome, too.

Few accounts linking Zika and the autoimmune disease have made it into the peer-reviewed literature. Last March researchers wrote in the journal Eurosurveillance that in French Polynesia the incidence of Guillain–Barré had increased 20-fold since Zika outbreaks began there in the past couple years—but no official data has yet been released. What’s more, Zika is prone to heavy mosquito-driven outbreaks. In 2007, the archipelago of Yap in Micronesia acquired the disease—and, shockingly, roughly 70 percent of its population was infected. (The nation’s total population was about 7,000.) But that outbreak also complicates the picture for the Guillain–Barré/Zika link: There did not appear to be a surge in cases of Guillain–Barré—or at least none that made it into official reports.

Yet certain facts do remain clear. Outside of the U.S. the incidence of Zika is becoming harder to ignore—boosting the chances that the U.S. could soon be faced with its own uptick. In just the past decade Zika has shored up its foothold in new territories. In the past two years more than 28,000 cases have been reported across French Polynesia. There is no routine testing for the virus and, like Ebola, the cases did not stop at those countries’ borders. Tourists have now brought cases back to Thailand, Germany, Japan, Australia and elsewhere. Then, in May 2015 public health authorities confirmed that Zika had also reached the Americas. Brazilian officials said that the disease had cropped up in the northeastern part of the country. Making matters worse, this pattern has the disturbing echo of familiarity: Before dengue began showing up in the U.S., cases had appeared in the Pacific islands and Brazil, too. The similarities are fueling concern among global health experts that the U.S. could face its own Zika outbreaks—the question is when.

Until recently cases of Zika were few and usually sputtered out quickly. The viral disease was first isolated in 1947 from a sick rhesus monkey in the Zika Forest of Uganda and only caused small outbreaks in Africa and Southeast Asia for more than 50 years. Yet the number of such cases has ballooned in the past couple years. The major reason: global travel. When the disease shows up in new populations that do not have any immunity to the disease it can more readily spread from person to person, at least as long as mosquitoes are around. There is no vaccine to protect against it or any cure.

Even as the disease becomes more pervasive, there are also few weapons left in our arsenal against it. The mosquitoes that transmit the virus from person to person typically bite during the day, rendering mosquito nets largely useless. Mosquitoes are also increasingly developing resistance to common insecticides. And the mosquitoes that carry the disease are widespread. “Anywhere with these vectors—Aedes aegypti mosquitoes and to some degree Aedes albopictus—could get this virus and have local transmission,” says Erin Staples, a medical epidemiologist and expert in mosquito-borne diseases at the U.S. Centers for Disease Control and Prevention. Right now that would be in the southeastern U.S. and into the Southwest, similar to where we currently may see cases of dengue or chikungunya, she says. Although travelers going to areas where Zika is common can help protect themselves by applying repellents that contain DEET, wearing clothes that cover their arms and legs, and using air-conditioning or window screens to try to keep bugs outside, none of those approaches are guaranteed—the American traveler with Zika had been wearing insect repellent with 30 percent DEET.

At home, there are some lifestyle factors that help protect against Zika. For one, in the U.S. many people do not spend much time outdoors or keep their windows open, helping prevent mosquito bites. “Transmission may be somewhat limited because of how we live our lives—going from air-conditioning at work to a car with air-conditioning so we may not be in the environment that much,” Staples says. But there is also much scientists do not know about the disease. For example, there is one reported case where the disease—at least circumstantially—appears to have been passed between humans via sexual transmission. A couple newborns in French Polynesia also tested positive for the virus within the first couple days of life, suggesting it may be possible for the virus to be passed from mother to child.

Primed for paralysis
The Guillain–Barré complication is just another wrinkle in an already formidable health problem: We do not have a true sense of how common Zika has become. “We don’t even know much about how far the virus has spread in Brazil. It may be in other parts of South America already but it won’t be detected unless blood samples are sent to a lab,” Weaver says. 

What’s the holdup? Zika virus itself could be detected in a patient’s blood within the first week or so of a patient’s illness (before the antibodies develop). But because the disease’s symptoms are so mild patients often do not seek immediate medical care. By the time patients show up and get blood work done, the virus is often no longer detectable and the antibodies that could be picked up by a CDC lab may look like those for the more common dengue.

Difficulties tallying Zika cases are more than a matter of inaccurate paperwork. Zika and dengue, for example, have different treatment plans. Dengue can lead to its more serious and life-threatening dengue hemorrhagic fever, where patients bleed profusely, so a dengue patient should avoid common painkillers like aspirin, ibuprofen and naproxen (Aleve) that could worsen the bleeding for patients. Yet with Zika doctors do recommend taking those painkillers to help with the fever and pain. And, longer-term, misdiagnosing Zika as dengue has another complication: Patients may not be on the lookout for the weakness that could signal the early onset of the associated autoimmune disorder; Guillain–Barré has no cure but there are several therapies they could tap that are known to help speed recovery—involving blood removal or injections of donor proteins.

As Zika becomes more widespread, the risk grows that an American traveler could bring it back to the U.S. and fuel a local outbreak or even—although much less likely—that infected mosquitoes may make their way overland to the U.S. For his part, the infected tourist likens his experience with Zika to a “tough flu that kicks your ass, makes your muscles sore and Advil barely made a dent.” It is not an experience he wants to repeat.