Up until the 1980s, human infections occurred across Africa and Asia, but those who contracted Zika typically only suffered mild illness. It wasn’t until 2007 that a large outbreak was recorded, this time in Micronesia. Up to three-quarters of the population of the Island of Yap were infected. “Like many arboviral agents, given the appropriate environmental and human conditions, new pathogens can be easily moved around the globe,” Powers said.

And that’s what Zika did. The virus began to ripple across the Pacific—and as it traveled, it seemed to change. In an outbreak in French Polynesia, in 2013, researchers linked the neurological disorder Guillain-Barré syndrome to Zika infections, according to a World Health Organization bulletin about the virus’s history. In May 2015, the first reports of locally transmitted infection in Brazil emerged. Because no Zika-endemic nations competed in the World Cup, held in Brazil in 2014, some scientists now believe Zika may have traveled from French Polynesia to Rio de Janeiro during the Va’a World Sprint Championship canoe race in August 2014.

In October last year, doctors noticed a spike in Brazil of cases of microcephaly, a birth defect that impedes brain development, and wondered if it could be linked to the uptick of Zika. By April, they had their answer: “Zika virus is a cause of microcephaly and other severe fetal brain defects,” the Centers for Disease Control and Prevention wrote last month. Since last spring, the virus has been linked to some 5,000 cases of microcephaly in Brazil alone.

“Here in Brazil, it is very clear, after the 2015 transmission season, that the epidemic is not over,” said Christopher Dye, the strategy director in the Office of the Director-General at the World Health Organization. “We are near-certain that Zika will reach all areas that are inhabited by the (supposed) principal vector, Aedes aegypti. Probably all countries in the Americas, except Canada. ... Maybe to the Mediterranean. It might move further north, through the range of Aedes albopictus, in the United States. Could there be other mosquito vectors?”

A crucial part of looking to Zika’s uncertain future means understanding its past. And as researchers trace the origin of the virus, they’re left with more questions than answers. For example, scientists still don’t know whether Zika is widely endemic in Africa and Asia, where it was found for decades before it traveled to Micronesia and French Polynesia en route to South America. They also don’t know the extent to which it may have caused birth defects and other severe health problems in the 20th century.

“Did [Zika] cause neurological disorders, but at a low and undiscovered incidence rate? Or did the virus somehow change, genetically or epidemiologically, as it crossed the Pacific?” Dye asked. “Could a new virus—if it is new—reinvade Africa and Asia and cause new and severe epidemics in places where infection was previously benign? All these questions are critical for public health, but we do not yet have answers.”