Swimming can be a great way to cool off during the summer. But even a pool that looks clean can be swimming with microscopic parasites, and that can cause a number of swimming-related illnesses, such as E. coli, giardia, campylobacter and norovirus.

The most common culprit of swimming-related illnesses is cryptosporidium or crypto for short. It’s spread through fecal matter and even a small amount can contain millions of germs.

“If a child has an accident in the pool or there’s any type of elimination in the pool, we know that we have to treat it right away,” said Vincent Groppa, who has been in the pool cleaning business for 35 years.

Regular levels of chlorine won’t kill crypto, which can survive in a well-maintained pool for up to 10 days. It’s also easy to catch.

The Centers for Disease Control and Prevention says that swallowing even one mouthful of water infected with crypto can lead to weeks of diarrhea, nausea and vomiting.

“You should emphasize to kids that they should not be swallowing the water, and they shouldn’t be going to the bathroom in the pool,” explained Catherine Roberts, Consumer Reports health editor.

To protect yourself and your family, make sure anyone who swims in your pool follows some simple guidelines.

Anyone who is experiencing diarrhea or has been sick should not swim. Have people take a shower or at least rinse off before they swim. Also have kids take frequent bathroom breaks to reduce the risk of accidents. That goes for babies too.

“Those swim diapers are not foolproof,” said Roberts.

If fecal matter does get into the pool, don’t take any chances.

“Get everybody out of the water and shut it down,” said Groppa.

Groppa says the only way to effectively kill crypto is to have a professional super-chlorinate the water and then slowly bring it back to normal levels again, a process that can take at least eight hours.

“It’s like a supergerm, you know, so that’s the only way to take it out of the pool,” he explained.

If you’ve gotten sick after swimming, your doctor can run tests to see if crypto is the cause. If it is, the CDC says you should wait a full two weeks after the diarrhea has stopped before you get back in a pool.