One day in late April, a caged rhesus monkey on a tree platform on the shores of Lake Victoria in Uganda developed a fever. Within two weeks, researchers isolated the virus that had sickened it. The illness, which had never been detected before, was dubbed the Zika virus, after the forest where it was first found. Researchers soon discovered that the virus was mosquito-borne, and that it could infect humans as well as monkeys.
Over the next half-century, researchers identified the Zika virus in humans just 14 times. Those cases were scattered across west and central Africa and Southeast Asia. This scarcity of recorded cases doesn’t mean the virus was sitting still: While there were no large-scale outbreaks observed in this period, researchers frequently found the antibody to the Zika virus in human bloodstreams in about two dozen countries. Those antibodies serve as evidence that a person was once infected with the disease, even if they are no longer contagious.
Not everyone with the antibody gets sick: Only one-fifth of those infected with Zika have symptoms. Those who do usually suffer from a rash, a fever, a headache, and achiness for several days — conditions easily confused with other ailments, such as the flu or an allergic reaction.
In April 2007, doctors on Yap, an island of less than 10,000 people in the Western Pacific, saw an influx of patients with flu-like symptoms accompanied by rashes. Researchers from the Centers for Disease Control and Prevention who arrived several weeks later went house to house taking blood samples. They came to the conclusion that about 73 percent of the population ages 3 and older had been infected with the Zika virus.
The outbreak marked the first time Zika was observed infecting an entire population — and the first time it was noted in a place with no possible monkey carriers. Until the Yap outbreak, the disease was wrongly believed to mainly infect non-human primates, and only occasionally infect people. Researchers have since learned that Zika can be carried by many mosquito species, including some that primarily bite monkeys and others that efficiently spread the disease among humans.
For the second time in six years, an island was hit with a Zika epidemic. This time, doctors noticed a strange correlation: Right after Zika hit, there was a spike in cases of Guillain Barré syndrome, a rare and mysterious autoimmune disease that can cause temporary paralysis.
From French Polynesia, the virus began island-hopping. French Polynesians visiting Easter Island — also known as Rapa Nui — for the island’s annual Tapati Festival, likely brought the virus with them, and mosquitoes then spread it to others. Soon, the virus made its way across the South Pacific to New Caledonia, Cook Islands, Solomon Islands, Vanuatu, Fiji and Samoa.
Some have theorized the Zika virus leapt to Brazil during a 2014 canoe racing championship in Rio de Janeiro. Others say the culprit was soccer -- either the 2014 World Cup, or a 2013 match that a Tahitian team played in. And still others say it’s unfair to blame sports at all – it could have arrived on any of the thousands of flights that land in Brazil from the Pacific or Southeast Asia each year.
What’s clear is that by February 2015, the mysterious illness had spread to six of Brazil’s 26 states. In May, the public health ministry confirmed it was Zika. The world’s health leaders issued advisories, but did little else initially to investigate or fight the outbreak.
That changed in September, when doctors noticed an uptick of microcephaly, a condition where babies are born with underdeveloped brains, precisely in the places where the Zika outbreak had raged. Within weeks, researchers found the Zika virus in the brains of miscarried babies -- the first of many clues that would link Zika and microcephaly. By the end of 2015, public health officials estimated Zika had infected more than 1 million people in the Americas, and Brazilian doctors were investigating microcephaly in nearly 3,000 babies.
By September, 48 countries and territories in the Americas and 10 countries in the Pacific, Asia and Africa had reported active Zika transmission, meaning mosquito populations in those places were spreading the disease. The CDC advised pregnant women to avoid traveling to Zika-infected areas, and to be tested for the virus if they have symptoms.
In the mainland United States, the disease has taken hold in parts of Florida, and hundreds of travel-associated cases have been identified elsewhere in the country. In some cases, the disease is also known to be transmitted sexually. The mosquitoes that carry the disease are found across a large swath of the nation, so health officials worry the disease could soon spread elsewhere.
As Zika continues to find a home in new countries around the world, research teams are racing to develop and test a vaccine. Meanwhile, public health officials are struggling to contain the disease and the mosquitoes that carry it. In Brazil, the government has deployed thousands of troops to fight mosquitoes door to door. In the U.S., funding to research and fight Zika has been stalled, caught up in a political fight in Congress.
