Nothing spoils a day at the beach like these two dreaded words: swim ban. Summer only officially started last week, and the Chicago Park District already has hoisted the red flag that renders beaches off limits for swimming 23 times. Trying to pinpoint the source of these bans often turns into an environmental whodunit. We know the weapon is E. coli, bacteria that are typically harmless but serve as a clue that fecal contamination — and the disease-causing microorganisms that go along with it — might be lurking nearby. When E. coli levels get too high, the "No Swimming" signs...