INDIANAPOLIS (WISH) — A nasty parasite is back in central Indiana, and it could be lurking in the pool you and your kids enjoy.

Cryptosporidium, often called “crypto”, is a parasite that can live in water for days, even in a pool with the proper amount of chlorine.

According to the Centers for Disease Control, it’s the leading cause of disease outbreaks in the United States connected to water.

Marion County has already seen 18 confirmed cases through the month of May. That’s 10 more cases than the same time last year.

The primary symptom of crypto is diarrhea that can last for several weeks.

“As little as 10 of those little parasites in a mouthful of water can knock you down for a couple of weeks. It just takes a little bit. So kids, you got diarrhea, keep them out of the pools,” said Mary Kay Foster, a registered nurse and Special Pathogens Program Manager with IU Health.

Experts say you need to be symptom-free for at least two weeks before going back in a pool.

“It can be challenging because people think they’re better, but we can still shed that parasite for up to two weeks after we’ve gotten over our symptoms,” said Jason Ravenscroft, the supervisor of the Pools, Septic and Wells Program with the Marion County Health Department.

The CDC reports more than 7,000 people have gotten ill from crypto in the last decade. But there are likely more cases that have not been reported because the person has to go to a doctor and get tested before the case is reported to the CDC.

Ravenscroft said pools with ozone systems or UV light systems kill crypto, but those systems are more expensive. Splash pads made after 2011 should also be safe because they’re required to have one of those systems.

Pools aren’t the only place where crypto can spread. The parasites are also linked to animals, especially cattle. So if you head to a fair or a petting zoo, make sure you and your kids wash with soap and water after you’re done.

Foster said alcohol-based sanitizer is not effective against crypto.

Got questions? Call the Department of Water Quality & Hazardous Materials Management at 317-221-2147.