From the New York Times:
"A swimming pool can offer relief from summer heat, but swimmers should know what they are jumping into. It could be a soup of nasty parasites.
Reports of gastrointestinal illness from use of public pools and water parks have risen sharply in recent years, according to the Centers for Disease Control and Prevention. The leading culprit is a microscopic organism that lives in human feces."
[. . .]
"'People should not swim or allow their children to swim when they have diarrhea, Ms. Hlavsa said. 'The water you swim in is shared with everyone,' she said. "So what one swimmer does has consequences for all the swimmers."