That clean, inviting swimming pool you’re jumping into this summer could be filled with a microscopic parasite that can live for days and wreck an immune system, federal health officials warn.

The Centers for Disease Control and Prevention published a report recently about the increased number of outbreaks caused by the fecal parasite, Cryptosporidium, more commonly known as “Crypto.”

The report shows a rise of 13% per year in tracked outbreaks of illness caused by the single-cell parasite, the leading cause of waterborne disease outbreaks in the country.

Health officials in the USA are warning people of a parasite, cryptosporidium, that can potentially cause chronic diarrhea. pic.twitter.com/TnGZuI6YBm

— Global Grind (@GlobalGrind) July 3, 2019

The outbreaks, which spike annually in July and August, are often attributable to public pools or water parks. Crypto is extremely tolerant of chlorine and can happily stay afloat in well-treated pools for more than seven days.

Swallowing water teeming with the parasite most commonly causes infection, so children are most at risk, the CDC notes.

Symptoms include nausea, cramps and diarrhea that can last weeks and lead to serious malnutrition and dehydration, the CDC says.

There were 444 outbreaks reported from 2009 through 2017, which resulted in nearly 7,500 illnesses, 287 hospitalizations and one death, according to the CDC report.

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