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No issue has garnered more attention than the Zika virus, which was first reported in Brazil in May 2015 and has spread rapidly within the country and to surrounding nations.
Though the virus merely causes fever, rash and joint pain in healthy adults, it's a significant risk to pregnant women because it's linked to the terrifying birth defect microcephaly.
Beyond the immediate health risk to those flooding into Rio for the Olympics, officials are worried that infected travelers could hasten the spread of the disease after the games.
In May, 150 global health experts called for the Olympics to be moved to a different country. But the World Health Organization has said the risk is not great enough to mandate such action.
There are mitigating factors. The Olympics will fall in the heart of Brazil's winter, meaning temperatures are forecast to be modest, rain is expected to be scarce, and the population of mosquitoes — the primary transmitters of Zika — should be on the low end.
Few athletes have expressed deep concerns about Zika, though some have said family members will avoid traveling to Rio because of the virus. The notable exceptions have been top golfers such as Rory McIlroy, Jordan Spieth and Jason Day, who all cited Zika as the reason they're skipping the games.
Critics, however, have argued these wealthy professionals never wanted to play in the Olympics in the first place.
The absence of those big names aside, the questions surrounding Zika and the Olympics likely will not be resolved until months after competition ends, when global health impacts will be more measurable.