As the weather warms up, health officials are sending out a warning to Hoosiers about a difficult-to-kill parasite found in public pools that can make them very sick.

The Centers for Disease Control said Cryptosporidium (Crypto) is a parasite that is transmitted through fecal matter from an infected person. It can cause “profuse, watery diarrhea that can last up to 2–3 weeks.”

The government agency reports outbreaks of Crypto doubled in the U.S. from 2014 to 2016 with 32 outbreaks linked to pools and water playgrounds last year. In Ohio, there were 1,940 reports of people getting sick in 2016.

In 2015, there were 188 cases of Crypto voluntarily reported in Indiana, but it is not known if those cases came from public pools or water parks.

Currently, water samples are being tested from public pools across Central Indiana. HML Lab in Muncie is conducting many of these tests.

Laboratory manager Jaima Ballentine said her staff finds all sorts of bacteria, even fecal matter swimming in the water. That bacteria can usually be wiped out with normal levels of chlorine, but Crypto is much more difficult to kill.

“Bacteria are usually killed within a minute or less of contact with chlorine. Crypto can actually last up to 10 days,” said Ballentine. “If you do have a pool that is infected like the CDC has been finding, you have to super chlorinate the pool and go through some extra procedures.”

Public pools are required to test their water every week. Ballentine said HML Lab has not been asked to test for Crypto yet, but she warns families should take steps to prevent an outbreak.

“You’d like to think people use common sense and don’t do this, but they’ll either go swimming shortly after a bout of diarrhea. They’ll let their kids go swimming after a bout of diarrhea,” she said. “If you soap and water your children after they do number two at the pool, it’s better to send them back into the pool that way.”