A lifeguard keeps watch on opening day of the newly renovated McCarren Park Pool on June 28, 2012 in the Brooklyn borough of New York City. The historic 37,000 square-foot pool had been closed since 1983 but has been rejuvenated by a $50 million restoration. Today is the first day New York City public pools opened for the summer. Mario Tama/Getty Images

Be careful when you go for a swim in the neighborhood pool this summer.

The parasitic infection cryptosporidium — known as crypto — is on the rise in the US, according to the Centers for Disease Control and Prevention. The parasite is the most common cause of diarrhea linked to swimming pools and water parks, and it spreads when people swallow something that has come into contact with the feces of a sick person, such as pool water.

The CDC says the infection can make "healthy people sick for up to three weeks with watery diarrhea, stomach cramps, nausea, or vomiting, and can lead to dehydration."

In 2016, there were at least 32 outbreaks in the country, double the number in 2014. In 2016, Ohio identified 1,940 people with crypto after observing as many as 571 cases annually between 2012 and 2015.

The crypto parasite is hard to kill — it can survive the standard levels of chlorine and other pool disinfectants that kill most other germs within a few minutes. To get rid of crypto, the CDC recommends closing pools after contamination for an hourslong period of "hyperchlorination."

Keeping crypto out of a pool also means relying on people to be responsible about showering before getting in and staying away from the water if they've recently had diarrhea. But that doesn't always happen — a recent survey by the Water Quality and Health Council found that 25% of adults said they'd swim within an hour of having diarrhea, and just over half said they didn't shower before getting in a pool.

The cryptosporidium parasite. CDC

In 2010, the CDC launched a DNA-fingerprinting tracking system for identifying crypto outbreaks.

To avoid getting sick, the CDC recommends not swallowing any water while you swim — easier said than done, of course, but it takes just a mouthful of contaminated water to make you sick.

Experts also urge people to rinse off before diving in and to have frequent bathroom breaks for kids. They also suggest changing diapers for young ones in a separate area away from the pool.

And if you've been sick with crypto? Stay out of the water, please.