The virus was discovered in 1947 in a feverish rhesus monkey living in the Zika Forest of Uganda, but until 2007 scientists knew of only 14 human cases of the disease. That year it arrived on the travel-brochure-perfect Yap Island in the southwestern Pacific Ocean. Within a few months, nearly three-quarters of the island's 11,000 or so residents older than 3 had been infected.
At first, those sick with Zika developed fever, joint pains and eye inflammation; then a red, bumplike rash erupted, sometimes followed by painful swelling of hands and feet. Some people vomited. Others were sensitive to light. But the symptoms usually resolved a couple of days later, and no one died.
In 2013 Zika popped up again, this time in Tahiti and other parts of French Polynesia. An estimated 28,000 people (about 11 percent of the population of those islands) felt sick enough with the virus to seek medical care. By 2014 it was showing up in several other South Pacific spots: New Caledonia, east of Australia; the Cook Islands; and, early this year, Easter Island, which marked the official arrival of the disease in the Americas, since that remote island is part of Chile.
Zika showed up in Brazil in May, prompting the World Health Organization and the Pan American Health Organization to issue an epidemiological alert. As of June 16, the number of suspected cases in the Brazilian state of Bahia topped 24,000. "The virus is now causing serious problems in Brazil," says Caio Freire, a microbiologist at the University of Sao Paulo.
So what happened? Why would an obscure virus emerge so suddenly and spread so fast?
Zika is a flavivirus related to yellow fever, chikungunya, dengue fever, West Nile and Japanese encephalitis, all of which are common in tropical climates but have started to spread in the United States and Europe in concert with warming temperatures, and they can cause serious health problems, even death. Yet Zika is a still a bit of a puzzle - for example, why, having been identified almost 70 years ago, did it get going among humans only recently?
One possibility is that the virus has evolved, either making humans more susceptible or making it easier for mosquitoes to transmit it. "These types of viruses can evolve very fast," Freire says. "The problem [with confirming that theory] is that we have very few complete genomes of this virus to study."
Researchers believe that Zika most likely was spread in the blood of an infected traveler - perhaps someone from Uganda or elsewhere in East Africa - who served as a meal for a local mosquito. When that insect latched on to someone else, a new chain of transmission began.
The virus spread quickly because people in the new location had no previous exposure or immunity. "When a new virus is introduced in a population which has never seen it before, it can be transmitted easily, explosion-like," says Dennis Tappe, a virologist at the Bernhard-Nocht-Institute for Tropical Medicine in Hamburg.
"This happens more or less by chance - we have more people who travel, bringing new viruses to non-immune populations," he says.
In a paper published in June, a group of Brazilian scientists suggest that Zika may have been brought into their country during the 2014 World Cup soccer tournament played in Brazil.
While Zika is usually mild, it can morph into Guillain-Barré syndrome, an autoimmune disorder that can cause paralysis, severe pain, loss of bladder control, difficulty breathing and sometimes death. And given how fast the disease spread in Yap and elsewhere, taking public health authorities by surprise, this is what has American public health experts especially worried.
"There seems to be a fairly convincing link emerging between Zika and Guillain-Barré syndrome," says Scott Weaver, director of the Institute for Human Infections and Immunity at the University of Texas Medical Branh. "In a couple of locations where there had been outbreaks, they've noticed an increase in an incidence of Guillain-Barré immediately afterwards. That's very concerning." The link has been seen in French Polynesia, where soon after the Zika outbreak, the incidence of Guillain-Barré went up 20-fold. In the recent outbreak in Bahia, 53 cases of the syndrome were confirmed, with one death so far.
Even a small number of cases of Guillain-Barré is a big problem, Weaver says. "People can take years to recover" from it.
What also adds significance to the Zika story is that the virus apparently can be transmitted not only by mosquitoes but also by sexual intercourse. Scientists believe there are no other vector-borne diseases that can spread this way.
The first suggestion that you can get Zika both from a mosquito and from your lover came in 2008, when an American scientist contracted Zika in Senegal and fell ill a few days after returning home to northern Colorado. Soon after, his wife also got Zika, even though she hadn't traveled anywhere far for months. Research published this year puts more weight on the suspicion that Zika is sexually transmitted: The virus has been found in semen.
Researchers say this does not mean that sexual transmission will take over from mosquitoes as the main way in which the virus is spread, although it would make prevention more complicated than wearing mosquito repellent. "It's an additional route of transmission that can contribute to an epidemic, but the effect will not be big," Tappe says. "If someone is sick, having intercourse is not very likely."
Although Zika has been identified in several travelers returning to the United States from the Western Pacific, there haven't been any locally arising cases, according to the Centers for Disease Control and Prevention. Not yet, anyway: The types of mosquitoes that spread Zika, dengue and chikungunya - Aedes albopictus, known as the Asian tiger mosquito for its distinctive black and white marks, and Aedes aegypti - are already present in the much of the eastern part of the country.
Once the vectors are there, Zika and other flaviviruses may follow. According to Marc Fischer, a medical epidemiologist with the CDC's Arboviral Diseases Branch, "The mosquitoes that transmit dengue, chikungunya and Zika viruses are found throughout much of the Americas. Therefore, Zika virus will likely spread to new areas, and people should take steps to prevent mosquito bites to reduce their risk of infection."
What do the experts advise? Wear a repellent when you go outdoors, fix damaged window screens and empty standing water from containers around your house that can become breeding grounds for mosquitoes.
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Zaraska is a freelance health and science writer and author of the forthcoming "Meathooked: The History and Science of Our 2.5-Million-Year Obsession With Meat."
