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ST. LOUIS COUNTY, MO – Two swimming pools in St. Louis County reopened Friday after being shut down by concerns over a parasite called cryptosporidium.

There’s been just one confirmed case of someone getting sick from it. The infection can give you diarrhea but most likely will not put you in a hospital, experts said. It can live in water for about 10 days.

The Webster Aquatic Center and one of the pools at the Shrewsbury Aquatic Center were closed Thursday. The child who got sick had been to both places, officials said.

“We worry about it because a lot of the outbreaks with cryptosporidium have been found in swimming pools,” said Dr. Matt Kuhlmann, an expert in infectious diseases for Washington University at Barnes-Jewish Hospital. “It’s because the parasite itself is able to resist the normal level of chlorine in pools that would kill off bacterial infections and other things that would make us sick.”

Pools had to hyperchlorinate – adding so much chlorine in order to kill off the parasite, he said. That also makes the pools unsuitable for swimming. With the parasite eliminated, the chlorine was adjusted to regular levels again.

Concerns remain about whether swimmers may have been infected before the case of cryptosporidium was confirmed. The infected swimmer was last known to swim in the three weeks ago. Infected people typically become sick within a few days.

“So if you were swimming in the pool earlier this week and you haven’t become ill by now, the chances of you becoming ill from an exposure is very minimal at this point,” Dr. Kuhlmann said. “The disease itself, if you catch it, goes away on its own without any treatment. You’re uncomfortable for a while, but it’s not anything that’s necessarily going to cause you to end up in a hospital.”

Health experts advise anyone with diarrhea should simply not swim. They should stay out until they’re symptom free for two weeks. People should take special care with children in diapers; no changing diapers poolside.

The illness can be serious for people with compromised immune systems.

“It can be very hard to eradicate the infection without improving the function of the immune system,” Dr. Kulmann said. “There are treatments available but they’re not as successful as antibiotics would be for a bacterial infection.”

It is spread by ingesting contaminated food or water. So it’s concern at home and not just at the pool. Good hand washing is key to prevention.