THURSDAY, May 18, 2017 (HealthDay News) — Families seeking to cool off don’t expect to pick up a nasty infection. Yet, outbreaks of a diarrhea-causing parasitic infection have doubled in recent years at swimming pools and water playgrounds in the United States, health officials warn.

At least 32 outbreaks of cryptosporidiosis were reported in 2016, compared with 16 outbreaks in 2014, according to a new report from the U.S. Centers for Disease Control and Prevention.

Crypto is the most common cause of diarrhea, the CDC says. It spreads when people come in contact with the feces of an infected person.

Otherwise healthy people can be sick for up to three weeks with watery diarrhea, stomach cramps, nausea or vomiting, the CDC warns. The infection can become life-threatening in people with compromised immune systems.

The cause? Adults or children sick with crypto-caused diarrhea are swimming in public pools despite their illness and further spreading the parasite, said Michele Hlavsa, chief of the CDC’s Healthy Swimming Program.

During a diarrheal incident, “a typical person who’s sick with Cryptosporidium can release 10 million to 100 million oocysts, which is the infectious stage of crypto,” Hlavsa said. “Swallowing 10 or fewer of these oocysts can make you sick. Looking at a typical-sized pool, even swallowing a mouthful of water can make us sick.”

People also can contaminate pool water with crypto through physical contact, said Lilly Kan, senior director of infectious disease and informatics with the National Association of County & City Health Officials (NACCHO).

For example, parents might spread the parasite if they change a child’s crypto-contaminated diaper and then hop in the water without properly washing their hands, Kan said.

Hlavsa explained that crypto is resistant to chlorine, and can survive up to 10 days in even properly chlorinated pool water.

“The oocysts have a hard outer shell, and that makes them very resistant to chemical disinfectants,” Hlavsa said.

Last year’s crypto outbreaks numbered the highest this decade, according to the CDC report.