It's just as gross as it sounds.

By Meghan Overdeep
July 02, 2019
Thomas Barwick/Getty Images

Sorry to be the bearers of bad news, but not even summertime dips in the pool are innocuous anymore. According to the Centers for Disease Control and Prevention, a fecal parasite that can be transmitted via swimming pools is on the rise just as we're approaching some of the hottest days of the year.

Cryptosporidium, or "crypto," is a hard-to-kill parasite that can resist the chlorine in swimming pools for up to seven days. It causes cryptosporidiosis, a profuse, watery diarrhea that can last up to three weeks. Hotel swimming pools and hot tubs can be a major source of these outbreaks, experts warn.

Between 2009 and 2017, 444 cryptoporidiosis outbreaks resulted in over 7,000 cases across 40 states. The number of reported outbreaks has increased an average of approximately 13% per year, the CDC reports.

In pools, crypto can enter the body when a swimmer swallows contaminated water. The water becomes contaminated when an infected swimmer transmits the parasite through lingering residue on their person or a diarrheal incident—which, gross as it may be, can be hard to detect in a pool full of kids.

"Diarrhea caused by crypto can be pretty watery so it can be pretty stealthy," Michele Hlavsa, chief of the CDC's Healthy Swimming Program, told NBC News.

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The CDC advises that parents of kids and babies who have had diarrhea recently keep them out of pools for at least two weeks after their diarrhea subsides. When a diarrheal incident does occur, the pool operator is supposed to clear the pool and flush it with extremely high levels of chlorine or bromine to kill the parasite.