There has been a rise in the number of people catching diarrhea from dirty swimming pool water, according to new statistics from the Centers for Disease Control and Prevention (CDC).

Twice as many people contracted cryptosporidiosis — a type of gastrointestinal illness caused by a parasite that can lead to watery diarrhea that lasts for up to three weeks — in 2016 compared to 2014. Since 2004, the number of annual cryptosporidiosis incidents has tripled in the U.S.

A person can get ill from cryptosporidiosis after being exposed to contaminated swimming pool water, water in water parks, drinking water and food, as well as from contact with people or animals who have been infected. The CDC said that at least 32 aquatic facility–associated cryptosporidiosis outbreaks occurred in 2016. As well as diarrhea, symptoms of cryptosporidiosis may include stomach cramps, dehydration, nausea and vomiting.

The public is advised not to swim or let their children swim if they have been sick with diarrhea. Health care providers have also been advised to instruct cryptosporidiosis patients not to go back into the water until they have been diarrhea-free for two weeks.

As even a single mouthful of contaminated water can lead to cryptosporidiosis, Michele Hlavsa, chief of the CDC’s Healthy Swimming Program, has recommended that parents encourage their children not to swallow water when swimming, as well as to avoid buying pool toys that may encourage swallowing, such as cups. “Take kids on bathroom breaks every hour, and check diapers in a diaper-changing area and not right next to the pool,” Hlavsa told CNN. “We all share the water we swim in, but we don’t want to share germs, pee or poop.”

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Write to Kate Samuelson at kate.samuelson@time.com.