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People who swim too soon after recovering from diarrhea — or even while they are still sick — are spreading a waste-borne parasite in public pools and water parks, federal health experts said Thursday.

Outbreaks of cryptosporidium doubled between 2014 and 2016, and three outbreaks from last summer demonstrate the problem, the Centers for Disease Control and Prevention said.

Outbreaks of a parasitic infection called cryptosporidium have doubled since 2014. CDC

“The parasite can spread when people swallow something that has come into contact with the feces (poop) of a sick person, such as pool water contaminated with diarrhea,” the CDC said in a statement.

“Crypto is the most common cause of diarrheal illness and outbreaks linked to swimming pools or water playgrounds because it is not easily killed by chlorine and can survive up to 10 days in properly treated water,” the CDC added.

“Swallowing just a mouthful of water contaminated with Crypto can make otherwise healthy people sick for up to three weeks with watery diarrhea, stomach cramps, nausea, or vomiting, and can lead to dehydration.”

Related: Number One Pool Problem? It's Number Two

Even proper chlorination of pools doesn’t kill the persistent parasite, the CDC said. It takes an especially large dose of chlorine to eliminate it.

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“Young swimmers aged under 5 years are more likely to contaminate the water because they are more likely to have inadequate toilet­ing and hygiene skills."

CDC pool safety expert Michele Hlavsa and colleagues gathered information on all recent reports of cryptosporidium outbreaks.

They said at least 32 outbreaks were reported in 2016, compared to 16 outbreaks in 2014 and 13 the year before.

Related: Here's the Poop on Your Pool Water

Hlavsa’s team reported on three outbreaks that made people sick last July and August in Arizona, Alabama and Ohio. In Arizona, 36 out of 51 Little League players and their families got sick at one Phoenix-area pool, and the whole county reported 352 confirmed cases of cryptosporidiosis.

At least 17 percent of the people who were sick admitted they had swum while they still had diarrhea symptoms, the CDC team said.

In Ohio, the average number of cases was just under 400 for many years but spiked to 1,940 cases in 2016, the CDC team said. More than 40 percent of the cases could be traced to pools or water parks.

And in Alabama, 23 people were confirmed to have cryptosporidium infections and as many as 35 got sick from one single venue.

Related: What Makes Your Eyes Red in the Pool?

“Young swimmers aged under 5 years are more likely to contaminate the water because they are more likely to have inadequate toilet­ing and hygiene skills; therefore, prevention efforts should focus on their parents,” the CDC team wrote in their report.

“As the Arizona outbreak investigation demonstrated, patients continue to swim while symptomatic,” they added.

All swimmers should shower and use soap before going in any swimming pool, the CDC notes. Toilet paper alone does not remove disease-causing germs.

“Health care providers should also instruct cryptosporidiosis patients not to go back into the water until they have been diarrhea-free for two weeks,” the team added.