People pee in swimming pools – that probably doesn’t come as a shock to you. You tuck it in the back of your mind when you go swimming; but you when you accidentally swallow a mouthful of pool water, you know you’re getting more than H2O and chlorine. Well, there’s another threat lurking in public swimming pools. It’s a diarrhea-inducing parasite called cryptosporidium, and federal officials said back in May that cases of the bug are on the rise.
Outbreaks of cryptosporidium (Crypto) doubled between 2014 and 2016, including three that occurred last year, according to the CDC. There were at least 32 outbreaks in 2016, compared to 16 outbreaks in 2014, and 13 the year before. 
In a statement, the agency said:
“The parasite can spread when people swallow something that has come into contact with the feces (poop) of a sick person, such as pool water contaminated with diarrhea.” 
Suddenly Netflix and A/C sound very inviting.
The CDC added:
“Crypto is the most common cause of diarrheal illness and outbreaks linked to swimming pools or water playgrounds because it is not easily killed by chlorine and can survive up to 10 days in properly treated water.
Swallowing just a mouthful of water contaminated with Crypto can make otherwise healthy people sick for up to three weeks with watery diarrhea, stomach cramps, nausea, or vomiting, and can lead to dehydration.” 
Don’t rely on chlorine to clean up the poop: Even proper chlorination doesn’t kill Crypto. 
The most heavily-affected state was Ohio, with 1,940 people being sickened by the parasite in 2016, compared to less than 600 in any previous year.
It’s not clear why Crypto cases are on the rise. The CDC said:
“It is not clear whether the number of outbreaks has increased or whether better surveillance and laboratory methods are leading to better outbreak detection.”
The agency wrote that the youngest swimmers are the most likely to infect pools with Crypto.
“Young swimmers aged under 5 years are more likely to contaminate the water because they are more likely to have inadequate toileting and hygiene skills; therefore, prevention efforts should focus on their parents.” 
Regardless of age, many people continue swimming even when they’re symptomatic.
Once a pool has been infected, Crypto spreads easily and is stubborn against efforts to eradicate it. In chlorinated water, Crypto can survive for up to 10 days, and it only takes a small gulp of water to become infected. The only way to rid a swimming pool of the parasite is to close the pool and treat it with extremely high, extremely toxic levels of chlorine. , 
Michele Hlavsa, R.N., M.P.H., chief of CDC’s Healthy Swimming Program, said in a statement:
“To help protect your family and friends from Crypto and other diarrhea-causing germs, do not swim or let your kids swim if sick with diarrhea. Protect yourself from getting sick by not swallowing the water in which you swim.” 
(Hopefully, if you’re an adult, this goes without saying.)
The CDC also offered these tips for staying Crypto-free this summer:
- If diarrhea is found to be caused by Crypto, wait 2 weeks after symptoms have subsided before going swimming.
- Rinse off in the shower before getting in the water to help remove any germs on your body that could contaminate the pool.
- Take children on frequent bathroom breaks, and check diapers in a diaper-changing area, not right next to the pool.
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Julie Fidler is a freelance writer, legal blogger, and the author of Adventures in Holy Matrimony: For Better or the Absolute Worst. She lives in Pennsylvania with her husband and two ridiculously spoiled cats. She occasionally pontificates on her blog.