WASHINGTON (AP) — While millions of soccer fans cheer or groan over World Cup matches spanning North America, health officials will be on high alert for germs.

A heat wave may be the most obvious health threat. But infectious diseases can spread in a crowd, and experts are set to scrutinize wastewater, hospital visits, even social media for any signs that an outbreak might be brewing.

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Measles, one of the most contagious diseases, is among the top concerns, sparking a warning this week from the Pan American Health Organization, PAHO. With a nearly six-week stretch of packed stadiums, bars and tourist sites in 16 cities, officials are on the lookout for a long list of infections, from the stomach bug norovirus to mosquito-borne dengue fever.

"This is truly a marathon," said Palak Raval-Nelson, Philadelphia's health commissioner.