There's a heated battle going on about the Summer Olympics in Rio de Janeiro. Nearly 200 scientists signed a letter to the World Health Organization last week, calling for the games to be moved because of the ongoing epidemic of Zika in Brazil.
But many health officials — including those at WHO — say having the games in Rio doesn't pose a big enough threat to warrant moving them.
So who's right?
To figure out whether Zika might be a big problem at the Olympics, there's one key piece of information you need: How many mosquitoes will be in Rio during the games?
That's exactly what epidemiologist Mikkel Quam has been working on. He used a mathematical model — and data from another outbreak in Rio — to estimate the chance spectators and athletes will get a mosquito bite for three weeks in August, when the games take place.
"I was legitimately surprised," says Quam, who works at Umea University in Sweden. "There's very little mosquito activity during the Olympics."
August is winter in Brazil. It's cooler and drier. So the mosquito population is way down.
Only about 4 percent of fans will get bitten at least once by a mosquito that could carry Zika, Quam estimates. The chance they'll catch Zika is even lower — much, much lower.
"I think we'll get cases but I don't expect many cases," Quam says.
It's hard to calculate the exact number. But a preliminary model suggests that, at most, 1 in 31,000 people at the games will get infected with Zika, Quam and his colleagues recently reported.
Officials are expecting around 500,000 spectators and athletes. Then the model predicts, there will be — at most — 16 cases of Zika at the Olympics.
So attendees are much more likely to get the flu or food poisoning at the games than Zika, the European Center for Disease Prevention and Control concluded.
"If I would be an athlete competing, from what I've read, I would be more concerned about the pollution in the water than Zika," says Alessandro Vespignani, at Northeastern University in Boston.
But whether the Rio games pose a danger to the world isn't just about the number of Zika cases, Vespignani says. It's also about where those cases go — what's the chance a fan or athlete brings the virus home to a place without Zika and triggers a new outbreak in Africa or Asia — or here in the States?
So Vespignani is working on a computer model for the U.S. government to predict how Zika will spread. Keeping the games in Rio doesn't seem to change the course of the epidemic in his models.
"There are already so many cases around the world that adding a little bit more cases is not going to make a difference at this point," Vespignani says.
So there's no reason to move the games because of Zika, he believes.
So far the continental U.S. has had about 600 cases of Zika. They've all come from travelers to other countries. Each year hundreds of millions of Americans travel to countries where Zika is circulating, a recent study found.
"The Olympics would represent less than 0.25 percent of all travel to Zika-affected areas," Dr. Thomas Frieden, the Centers for Disease Control and Prevention's director, said last week to reporters.
So even if the Olympics were called off, "we'd still be left with 99.75 percent of the risk of Zika continuing to spread," Frieden said.
But all these predictions and models are just that — predictions. Like weather predictions, they are often wrong. And they're based on many assumptions, such as the idea that Zika behaves similarly to the way other mosquito-borne viruses do.
"The problem is, we just don't know that," says Arthur Caplan, a bioethicist at New York University. He's one of the researchers that wrote the letter to WHO, calling for the Olympics to be moved. He says the stakes are too high.
"I think it's ethically dubious to run the Olympics when you've got an epidemic of a virus that we don't understand very well," Caplan says.
For instance, scientists still don't know how long Zika can linger in the body or how big of a problem sexual transmission is.
So Caplan says, why not not err on the side of caution — especially considering the devastating effects Zika can have for mothers and their babies.