It's called Cryptosporidium or crypto, and it is spread by feces.
The CDC says they are seeing more and more people come down with it.
A report released on Friday says that there's a nearly 13 percent increase every year between 2009-2017.
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In 2016, Harris County officials told ABC13 that there had been an increase of people getting sick from the parasite. Fourteen cases were reported that year.
"Even though the pool is chlorinated, it may not kill all the cryptosporidium," said Infectious Disease Dr. Michael Chang at Children's Memorial Hermann Hospital / UT Health. "It likes warm weather. It grows better in warm water."
The parasite is spread when children or adults who are infected defecate in the water. The parasite breeds. Those swimming in the contaminated water get the parasite when they ingest the water.
Patients who become ill with the parasite experience diarrhea for weeks along with pain in the abdomen and a low-grade fever. People with weakened immune systems can become severely ill and could die.
"The splash pad can actually be a big source of Crypto outbreaks because when you go to the splash pad you don't necessarily think about how that water is treated as it comes in. Actually a lot of splash pads, the water is recycled," said Dr. Chang.
Pool keepers are told to chemically shock any pool that is suspected or is contained with Crypto.
Parents are urged to avoid taking their children to the pool if they are sick with diarrhea. Swimmers are asked to swim with their mouths closed.
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