Stadiums quaked as U.S. soccer fans urged their men’s national team toward their successful qualification for the upcoming FIFA World Cup. Many of these supporters will soon descend on Brazil. There they will be joined by an estimated 600,000 revelers from around the globe. Unfortunately, these travelers may catch far more than the beautiful game.

To forewarn tourists of disease threats in Brazil, a new study led by Harvard University scientists has assessed the illnesses most often contracted during journeys to the South American nation. Researchers studied the case reports of 1,600 people who became sick after returning from Brazil between 1997 and 2013 to offer a snapshot of what people might soon encounter at the World Cup and the 2016 Summer Olympics.
Three big clusters of disease emerged: Feverish blights such as dengue virus and malaria are no strangers to the tropics, yet they landed in third place on the list. Traveler’s diarrhea—that familiar spoiler of overseas trips—was second.
The unexpected winner was skin worms, which accounted for two of every five recorded diagnoses among voyagers. “We were a little surprised by how common these skin infestations were,” says Harvard’s Mary Wilson, a global health specialist who headed the study. “But then if you consider that many cities visited by people are right on the coast, it makes sense.”

The leading skin worm was cutaneous larva migrans (CLM), a hookworm typically caught while hanging around beaches. The worm larvae live in sand and can penetrate the intact skin of a bare foot or an exposed bum. The major source of CLM is dog and cat poop littered on the seashore. One survey of a São Paulo district discovered the parasite in 90 percent of canines and felines whereas a separate inspection of Recife’s Alto Beach, a popular tourist destination, found the worm’s larvae in one of every three sand samples. Rather than discourage beach-goers, however, Wilson and her colleagues hope that the findings encourage people to wear proper footwear and avoid sitting on bare sand.

The report is useful for doctors back home, too, says Susan McLellan, a clinical professor of public health at Tulane University who was not involved with the study. “Family-practice doctors miss CLM all the time or mistake it for another kind of worm,” McLellan says. “The article provides a nice review of the infections that might arise during these mass gatherings.” The survey comprised health data from the GeoSentinel network, a collective of health clinics spanning 40 countries and six continents whose purpose is to measure maladies as they cross international borders.

Mosquito-borne dengue virus, malaria and yellow fever are constant concerns for Brazilian public health officials. Other fears for the World Cup surround the emergence of chikungunya virus, which is carried by the same mosquitoes as dengue and which also causes fevers and rashes. The virus first appeared in the Western Hemisphere in December 2013, and experts have warned that the World Cup might increase the possibility that a new catastrophic epidemic will spread across the Americas.

Four of the 12 World Cup stadiums are located in regions endemic with yellow fever or malaria (see map); the risk of dengue is high throughout the country. Dengue virus and malaria caused the most hospitalizations among the surveyed travelers. No cases of yellow fever were recorded in the study, but the U.S. Centers for Disease Control and Prevention advises vaccination for those attending inland matches in yellow fever hotspots. Immunizations for measles and influenza are recommended, too, given that these infections thrive in crowds. The agency’s website also features a list of health-related Portuguese expressions for visitors to learn, including phrases such as, “I have been bitten by mosquitoes” (Fui picado por pernilongo).

Yet tourists may find it more useful to study the mosquitoes’ behavior. Mosquitoes with malaria live near the Amazon Basin and bite at night. In contrast, the main species harboring dengue and yellow fever—Aedes aegypti—feeds during the day and populates urban areas. They lay eggs in the small puddles of water found in discarded plastic cups, used tires and flowerpots. Outdoor spraying of insecticides has minimal impact, given that Aedes aegypti mosquitoes tend to camp indoors. “The Brazilian Ministry of Health has worked hard to get everyone to empty standing water from homes,” Wilson says.

Dengue has no vaccine or cure, but bug repellents serve as great shields. Wilson adds that luckily the soccer tournament and Olympic games will occur during austral winter, when dengue incidence is lowest for Brazil (see graph).

Sexually transmitted infections (STIs) are another health risk for tourists. Nearly one out of five international travelers engages in casual sex while abroad, according to a review of health literature, and about half of these encounters don’t involve the use of condoms or other forms of protection. Approximately a dozen GeoSentinel patients were diagnosed with STIs on their return from Brazil. Brazilian authorities have recently cracked down on its sex worker industry, fearing a possible boom in STI incidence spawned by the games.

The aggressive tactics might be unwarranted, however, because mass gatherings like the World Cup and the Olympics are not historically connected with surges in prostitution or the trafficking of sex workers. “Sensationalism around international sporting events and paid sex is unnecessary,” says Marlise Richter, a sexual health and advocacy specialist at Sonke Gender Justice in South Africa.

During the 2010 World Cup in her home country, Richter and her colleagues interviewed over 2,000 male, female, and transgender sex workers at four sites. They didn’t find any increases in the demand or supply of sex work nor did they observe an influx of migrating prostitutes. The most important action that the authorities could take to limit the spread of STIs, Richter says, is to promote the use of male and female condoms. “People should be having safer sex whether it's with a sex worker, a girlfriend or a one night stand,” she adds.

So pack a pair of flip-flops, some bug repellant—and perhaps a box of condoms, too. Viva Brazil!