How mass events like the World Cup help spread disease around the globe

Up until recently, Zika was a little-known virus mostly confined to small islands in the Pacific Ocean and a few African nations. So how did it come to spark an "explosive" world health emergency, infecting more than 1 million Brazilians and many more throughout the Americas?

While health experts are unsure of the exact route Zika took, one hypothesis is that it arrived in Brazil during the 2014 World Cup.

The idea is that an infected traveler or athlete came to Brazil, along with thousands of others, for the World Cup in 2014. Then, presumably, a mosquito bit the traveler, contracted the virus, and went on to bite other humans, spreading the infection. Officials confirmed the first case in the country in May 2015. Since then, the virus — which is thought to cause birth defects — has infected 3 million to 4 million people throughout the Americas.

If so, this wouldn't be the first mass gathering that led to a disease outbreak. When thousands of international travelers gather in one place — whether for the World Cup, the Olympics, or Saudi Arabia's Hajj — germs can spread quickly. Research has documented this pattern over and over again.

And that doesn't bode well for the coming year. Brazil is also hosting the 2016 Summer Olympics. It's reasonable to worry that just as Zika found routes into Brazil, it will find ways to leave the country with travelers and start new epidemics across the world.

Mass gatherings are hotbeds of disease. This is (mostly) unavoidable.

There are several scenarios in which mass gatherings like the World Cup (or the Olympics, or the Hajj, etc.) can lead to major disease outbreaks:

1. Foreign infectious diseases can enter the host country via travelers and spread. This, presumably, is what happened with Zika.
2. Alternatively, the host country's native diseases can infect foreign travelers who aren't used to those germs.
3. Compounding the problems, poor sanitation can allow diseases to spread quickly throughout the event.
4. Finally, the germs that people catch at these events can spread around the world when everyone goes home.

Apart from No. 3, it's nearly impossible to eliminate these risks altogether.

Viruses, bacteria, fungi, and other germs are microscopic, so they can slip through borders undetected. And though countries can mandate vaccinations for travel visas, no amount of screening can stop all pathogens at the airport.

The World Health Organization acknowledges this: "Even when a host community’s existing health and other support services are adequate to deal with the regular disease burden affecting its own population (including occasional outbreaks), the influx of large numbers of people … can place a severe strain on such services," the organization writes.

Nowhere is this more true than in Saudi Arabia, where the yearly Hajj pilgrimage brings together millions of people across the world, many of whom come from poor countries.

The Lancet: Infectious Diseases

It's a perfect storm for the spread of disease: "Many [pilgrims] will have had little, if any, pre-Hajj health care, added to which are the extremes of climate and crowding, rugged terrain, mingling of populations from around the world, and migration into the country of livestock, butchers, and abattoir workers," The Lancet: Infectious Diseases reports. (Overcrowding at the Hajj poses other dangers as well. Last year, more than 2,000 died in a stampede).

Outbreaks that start in Mecca then find their way across the world. In 1987, a large outbreak of meningitis "rapidly spread to Hajjis of all nationalities participating in the pilgrimage and to the Saudi indigenous population," the journal Clinical Infectious Diseases reports. "Outbreaks were eventually reported in other Persian Gulf states."

And even though Saudi authorities have since mandated that pilgrims receive meningitis vaccinations, outbreaks still have occurred. An outbreak in 2000 led to meningitis infections in nine European countries, Emerging Infectious Diseases reports.

Emerging Infectious diseases

Even events that bring together relatively wealthy tourists are prone to disease outbreaks. There was an outbreak of the flu at the 2002 Winter Olympics in Salt Lake City — though a report in Emerging Infectious Diseases points out that its spread was slowed by a prompt response. Likewise, a measles outbreak followed the 2010 Olympic games in Vancouver (though that outbreak also didn't spread far).

Sometimes it's the event venue that's at fault. Nearly half the 12,700 attendees of the 1987 meeting of the Rainbow Family of Living Light (i.e., hippies), came down with shigellosis, an intestinal disease, because the water was contaminated. At the 1992 Glastonbury music festival in the UK, 72 people came down with campylobacter — which causes severe diarrhea — after drinking unpasteurized milk.

What can be done to stop Zika (or other diseases) from spreading at the Olympics?

With Brazil set to host the Olympics this summer, there are reasonable worries that Zika could spread further across the globe. So what can be done to stop that?

With any disease outbreak, surveillance is key. If a disease outbreak is identified early enough, event officials can distribute the right medicines, place people in quarantine, or identify the source of contamination.

During the London 2012 Olympics, the British Health Protection Agency rolled out a network for monitoring emergency rooms and doctors' offices for cases of infectious diseases. The HPA reports it was able respond to several cases of norovirus (stomach virus) among athletes, and identified Legionnaires' disease in a hotel. No major disease outbreaks occurred during the games.

There are already some concerns that Rio is unwilling or not prepared to engage in comprehensive disease surveillance. In October, the Associated Press reported that the Rio organizing committee decided against testing for viruses in the waters where the Olympic's aquatic events will occur, even though independent investigations found "dangerously high" levels of sewage.

Zika is a bit different from diseases like the flu, meningitis, and the measles. You can't catch it from water or air or from person-to-person. (As far as scientists know — there's some small evidence it can be sexually transmitted in men.) It only spreads from mosquito to human and vice versa, which makes it harder to contain. So if Brazil can eliminate the mosquitos in the Olympic areas, the threat will be diminished. But that also means if a sick traveler returns home to a country where the mosquitos live, it can spread there too.

Olympic officials have promised venues will be inspected on "a daily basis in order to ensure that any puddles of stagnant water — where the mosquitoes breed — are removed, therefore minimizing the risk of athletes and visitors coming into contact with mosquitoes," Reuters reports.

The games are set for August, when it will be winter in Rio (as it is in the Southern Hemisphere). Cooler drier conditions will keep mosquitos at bay. As will the massive amounts of mosquito repellent and insecticide that will bathe the city's athletic venues.

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