“Human Zika virus infection appears to have changed in character while expanding its geographical range,” the WHO paper concludes. “The change is from an endemic, mosquito-borne infection causing mild illness across equatorial Africa and Asia, to an infection causing, from 2007 onwards, large outbreaks, and from 2013 onwards, outbreaks linked with neurological disorders.”
“If Zika didn't produce any of these, we would not be talking about this issue now,” says Marcos Espinal, the director of communicable diseases for the Pan-American Health Organization, the WHO’s regional office for the Americas. “Zika, in most cases, is a very mild disease.”
Which means the problem is not the suffering inflicted by the initial infection; there probably won’t even be any suffering for most, since four out of five people never experience any symptoms. The danger is what might happen later—the fetuses that get microcephaly, the people whose brains and spinal cords swell and possibly sustain damage, the people temporarily paralyzed by Guillain-Barré, the ones who die because the Guillain-Barré stops their breath (as it can sometimes do) and they’re unable to get to a hospital with a respirator.
Zika is an epidemic on delay, with the worst of the outbreak’s effects trailing in the wake of the mosquitoes that carry the virus. In Brazil, the first reports of “an illness characterized by skin rash” appeared in March of 2015, according to the WHO; the first reports of neurological problems appeared in July. Until those neurological symptoms show up in a population, Zika presents a lot like its viral family members dengue and chikungunya. And there was no commercially available diagnostic test for Zika then, so when it first appeared, there was no good way to know that the country was seeing something new.
“It makes it harder to track, that’s for sure,” says Peter Hotez, the dean of the National School of Tropical Medicine at Baylor College of Medicine. “We weren’t really able to track the course of Zika in Brazil until we saw this big cluster of microcephaly cases. That means that we have to be proactive and preventive once we know where the virus is or where it's going. You can’t wait for the neurologic sequelae to show up, because by then the horse has left the barn.”
There’s a veritable stampede now in Brazil, with more than 7,000 cases of microcephaly or central nervous system malformation reported between October 2015 and early April 2016. And as the summer approaches, the Aedes aegypti mosquito is expected to gallop north, with Zika riding along.
“What are we doing right now as the virus is moving into Haiti, where it’s going to decimate Haiti, and as it moves into the Gulf Coast?” Hotez asks. He recently wrote an op-ed in The New York Times cautioning that for the Gulf Coast of the U.S., Zika could be “a catastrophe to rival Hurricane Katrina” if preventive measures aren’t taken.