They're called recreational water illnesses and include
"Getting sick in a pool is less likely than it used to be because people are very careful about that. There are very strong regulations about how often you have to have the water checked," said Dr. Lara Danziger-Isakov with Cleveland Clinic Children's Hospital.
But infectious disease experts warn that chlorine doesn't kill everything.
The most common water illness is caused by Cryptosporidium, it's a germ that causes diarrhea and can survive for days, even in properly chlorinated pools.
"I think people need to be very careful at home as well that they have the proper balance of chlorine in their pools," adds Dr. Danziger-Isakov.
But chlorine doesn't kill germs instantly.
And here's a fun fact. People have on average 0.14 grams of feces on their bottoms, which can contaminate recreational water. And just one dirty diaper or person sick with diarrhea can easily contaminate a large pool.
So needless to say avoid swallowing the water as best you can.
And if you're planning to hit the beach, think twice if we just had a big storm. Pollution in beach water is highest right after a rainstorm because of overflowing sewage pipes, and rainwater run-off--picking up pesticides and waste along the way.
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