Health officials will be on high alert for germs as millions of soccer fans gather for World Cup matches. A heat wave may be the most obvious health threat. But infectious diseases can spread in a crowd. Measles is one of the most contagious diseases and is a top concern. But with weeks of matches spread across sites in the U.S., Mexico and Canada, officials are on the lookout for a long list of possible infections. They'll use a variety of tools, including sophisticated wastewater tests and tracking hospital and doctor visits, to sound an alarm if there's any concern an outbreak is brewing.