From a New York Times article about summer swimming:

What the Centers for Disease Control suggests to combat the parasite is increased vigilance by pool staffs and swimmers. Thus the agency's brochure "Fecal Accident Response Recommendations for Pool Staff: What Do You Do When You Find Poop in the Pool?" and its poster that looks like an eye chart. It poses the question, "Can you read this?" The letters of the chart say: "Have diarrhea? Don't swim."