August is the best time of the year to take a dip in Lake Michigan, when its waters hover in the balmy upper 60s. Experts say so , and Chicago’s crowded beaches offer proof. But an invisible hazard can quickly turn a sunny day out into a sick night in.

In 2024, over 300 beaches across the Great Lakes closed to visitors or issued swim bans or advisories due to the presence of bacteria in the water — mostly E. coli, from nearby surface runoff or sewer system overflows, especially during heavy rain — according to state and federal data.

Bacteria levels triggered 83 advisories or closures in Illinois last summer, making it the second worst in the Midwest, with 71 in Lake County’s 13 lakefront beaches and 12 across nine beaches in Cook County. As of Thursday, Lake County beaches have had

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