A sitting duck. That's what South America was a few years ago when Zika first struck. The continent had never recorded a case of the mosquito-borne virus. And everyone was susceptible.
"So you get this huge raging epidemic that blows through the population, usually very fast and infects a pretty high percentage of the population," says Justin Lessler, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health.
That's exactly what happened. Millions of people across South and Central America have been infected with Zika. More than a thousand babies have been born with birth defects caused by the virus.
But now the worst might be over.
Lessler and his colleagues report evidence Thursday that the epidemic has peaked and started to subside. The virus has spread so quickly and infected so many people that it's actually burning itself out, the team writes in the journal Science.
By graphing the number of Zika cases over time, it easy to see cases have been declining rapidly over the past few months. But Lessler and his team couldn't tell if this decline was because the epidemic is waning or because the mosquito season is South America has finished for the year.
To figure that out, Lessler and his team built a simple computer model of Zika spreading across the Western Hemisphere. The model predicts the epidemic will probably last another 1.5 to 2 years and will likely peter out by spring 2018.
Then Zika will lay low for years, the team predicts. There could be small outbreaks, but "the disease is going to appear for all intents and purposes to have gone away," Lessler says. "It could be five years or even a decade before we have another significant epidemic."
The reason for this is pretty simple: A large portion of the population in Latin America will have been exposed to Zika and become immune to the virus.
"You can think of it like the forest after a giant fire," Lessler says. "All of the extra brush is gone, and it's going to take a while before enough brush — or in the case of a disease, enough susceptible people — can build back up to support another fire."
Clearly this is good news for South America. But what does it mean for the U.S.?
"That's an interesting question," says Bryan Lewis, a disease modeler at Virginia Tech.
The new study suggests Zika will be a threat in the U.S. for a few summers to come, Lewis says. But this summer will likely be the worst in terms of the number of imported Zika cases we get."Because the spike in Latin America is probably the highest it's going to be this current season," he says. "This summer is probably the big one."
All these predictions need to be taken with a big grain of salt, cautions epidemiologist Alex Perkins at the University of Notre Dame, because they are based on computer models — which aren't great at making long-term forecasts.
"It's much like when you watch the nightly weather," he says. "The weather people usually do a very good job of saying what the weather is going to be like tomorrow. But the longer time horizon you look at, the harder it is to predict what's going to happen."