When masses of people from around the world descend on a city for the Olympics, public health experts get a bit jittery. It's easy to understand why: Thousands of tourists cramped into stadiums and bars inevitably swap bacteria and viruses. In the past, this has led to outbreaks of measles and norovirus.

This year, an extra layer of anxiety has shrouded the Summer Games in Rio de Janeiro: Host country Brazil is a hot zone for a Zika virus epidemic that has now reached more than 60 countries.

While most people who are infected with the virus have no symptoms, Zika can cause devastating birth defects in fetuses and potentially lead to neurological problems in adults. It's also a relatively new pathogen in this part of the world, and there's still a lot researchers don't know about it.

Zika fears have prompted all kinds of extreme pronouncements about the games. A Canadian law professor, writing in the Harvard Public Health Review, called for moving or delaying the Rio Olympics this year because of the risks the virus poses. Some athletes, including the world’s third-ranked golfer, Rory McIlroy, have even said they're considering dropping out of the games because of Zika.

But this is all very likely misguided. According to the best available evidence, the chance of any visitor to Brazil catching Zika at this year's Summer Olympics is low. In fact, Zika is not even the infectious disease most likely to be an issue in Brazil later this summer. That's why the World Health Organization declared on June 14 that there's no reason to move or delay the games. Here's why the Olympics aren't expected to significantly increase the spread of Zika.

August isn't mosquito season in Rio

The main way Zika is spread from person to person is through mosquitoes, which act as a vector for the virus. Researchers say one reason Zika is unlikely to be an issue in Rio is because August there is cooler and dryer — and inhospitable to mosquitoes. Mosquito season there runs during its (Southern Hemisphere) summer, from about November or December to March. These are also the months with the most rainfall.

"It's going to be winter [during the Olympics], so the risk is going to be low in terms of mosquito transmission," said Duane Gubler, a leading researcher on mosquito-borne diseases at Duke-NUS Medical School.

Rio also happens to be far from the epicenter of the Zika outbreak in northeastern Brazil (where most of the confirmed Zika cases originated). And people who travel to Brazil for the Olympics are likely to spend most of their time in Olympic venues in Rio that have already been heavily treated with insecticide to control mosquito breeding.

For all these reasons, the European Center for Disease Prevention and Control (ECDC) said in its most recent Olympics risk assessment that there's a "very low risk" of travelers catching Zika in Rio this year — a conclusion echoed by the WHO on June 14. On July 13, the CDC also said that the worst-case scenario of Zika contagion stemming from the Olympics would likely only lead to a small number of cases, if any, in four of the 206 participating countries.

If dengue and the World Cup are any guide, there will be very few cases of Zika

When the World Cup was held in Rio in July 2014, dengue, another mosquito-borne virus that is transmitted by the same mosquito that transmits Zika, was expected to be a problem. Brazil has one of the highest rates of dengue infection in the world.

Yet, Gubler pointed out, "There was little or no dengue transmission that occurred in those areas." To be precise, of the million foreign tourists who went to Brazil for the tournament, only three contracted dengue — all of them in Belo Horizonte, a city that's about 300 miles north of Rio.

Because of the similarities between these two viruses, researchers are using the experience with dengue as a guide for Zika. And they've come to some pretty reassuring conclusions.

An estimated one or two tourists could get Zika during the three weeks of Olympic Games "Scientists have also done modeling predictions of the likely number of Zika virus cases based on that experience with dengue," explained John McConnell, editor of the journal Lancet Infectious Diseases. "The worst-case scenario is that there will be 3.2 Zika infections per 100,000 tourists. The much more likely scenario: Zika will affect 1.8 people per million tourists."

In other words, an estimated one or two tourists could get Zika during the three weeks of Olympic Games. (You can read the paper here.)

Remember, though, in the vast majority of cases, Zika doesn't seem to harm individuals who get it. About half of people who get infected experience no symptoms at all. The main risk of Zika complications is in fetuses. For this reason, Gubler said, "Reproductive-age females and males should take precautions," like wearing insect repellent and using condoms during sex, since in rare cases Zika can be sexually transmitted.

Otherwise, there's no need to stay away from the games because of Zika, Gubler said. (The WHO, Centers for Disease Control and Prevention, the CDC, and the ECDC have all said the same and suggested pregnant women avoid travel but everyone else simply take all the standard precautions.)

As for Zika's other potential health risks — like the neurological condition Guillain-Barré syndrome — the chances of Zika-induced complications are also extremely low, McConnell wrote in a recent Lancet commentary about the Olympics.

Travelers to Brazil are more likely to be sickened by flu or food poisoning than Zika

Although Zika is a novel and exotic virus, it won't actually be the biggest threat to health at the games.

The ECDC ranked the risk of various health threats during the Olympics. Interestingly, while it's classified Zika as posing a "low risk," gastrointestinal infections (caused by everything from food poisoning to norovirus, salmonella, E. coli, and Campylobacter) are considered a "moderate risk." Influenza, meanwhile, is "high risk."

That's because it's flu season in Brazil. Thinking rationally about these health risks, McConnell added, "People are much more likely to go home carrying flu than Zika virus, and flu is a much more dangerous disease." The flu kills more than 30,000 people a year in the United States. Zika hasn't killed any Americans.

One could make an argument that since there's a lot we still don't know about Zika, based on the precautionary principle the games pose too great a risk to human health.

But if you follow that logic, McConnell pointed out, travel to the country should have been banned long ago.



Hundreds of thousands of tourists have been going in and out of Rio every month since the Zika outbreak was declared and during the height of mosquito season. So far, however, countries including the US have been advising pregnant women and those considering getting pregnant soon to avoid travel to Zika-affected areas, but have not called for travel restrictions.

"Travel to the Olympics would represent less than one-quarter of 1 percent of all travel to Zika-infected areas"

What's more, according to the experience of cities that recently hosted Olympic Games, "the absolute number of visitors changes little compared with previous years, because regular tourists and business travelers are discouraged from visiting during the games," McConnell wrote in his commentary.

More specifically, the CDC's Tom Frieden estimates, "Travel to the Olympics would represent less than one-quarter of 1 percent of all travel to Zika-infected areas." He added, "So even if ... the Olympics weren't to happen, you'd still be left with 99.75 percent of the risk of Zika continuing to spread."

In the rare chance that an Olympic traveler catches Zika and exports the virus to another country with mosquitoes that could spread it around, Scott Weaver, a leading Zika specialist and the director of the Institute for Human Infections and Immunity at the University of Texas Medical Branch in Galveston, says there's likely little reason for worry: "It is likely that rates of human immunity in many other parts to the world that are permissive for circulation, such as tropical Africa and Asia, are already high enough to preclude major epidemics like we are seeing in the completely naive populations of the Americas."



That's because, unlike in Brazil, Zika had been circulating in these parts of the world for decades, and researchers believe people in those areas developed antibodies to fight the virus.



To summarize: The Olympics aren't expected to lead to a sudden surge in tourists in the country. And the tourists who do come are at extremely low risk of getting the virus and exporting it to other places. So let the games go on!

Watch: The Zika virus, explained