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At a Glance Outbreaks of a parasitic disease have risen roughly 13 percent per year.

Cryptosporidiosis, known as crypto, causes diarrhea that can last for weeks and puts the immune deficient at risk.

At least 7,465 people were infected with crypto in 444 outbreaks from 2009 to 2017.

More than 35 percent of all outbreaks and 55 percent of all infected individuals during this time came from recreational swimming sites.

Outbreaks of a parasitic disease that peaks in summer, thrives in swimming pools and water parks and is highly resistant to chlorine are on the rise.

This is according to a new analysis by the CDC , which detailed more than 400 cryptosporidiosis, known as crypto, outbreaks from 2009 to 2017, in which a total of 7,465 people were infected across 40 states and Puerto Rico.

The crypto parasite, more scientifically known as cryptosporidium , is the catalyst for the disease, which causes a gnarly bout of diarrhea that can last for weeks, nausea, vomiting and fever. It can even be fatal for those with weakened immune systems.

"It's really a problem because the dose to get infected is so small and the amount that can be excreted is so many many times larger," Dr. Radhika Gharpure, an epidemic intelligence services officer with the CDC, told weather.com.

The total number of outbreaks is likely higher than the actual count reported, the CDC says, adding that outbreaks – two or more infected individuals linked to a single place – of the disease have risen roughly 13 percent per year.

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The rise is in outbreaks might largely be chalked up to better testing methods for the parasite. In 2015, labs across the country started to use more comprehensive tests for crypto and other diarrhea-causing parasites, bacteria and viruses, according to Gharpure.

Swallowing water teeming with the parasite most commonly causes infection, so it's no wonder children are most at risk, the CDC notes.

Crypto doesn't just come from swimming pools and water parks, but it's where you're most likely to contract it. In the study, such sites accounted for more than 35 percent of all outbreaks and more than 56 percent of all the infected people. It's also the reason crypto outbreaks and individual infections peak in the warmer months of June to September, when more people are swimming.

"The point to emphasize is this is a really tough bug to kill," said Gharpure. "Crypto has this outer shell that allows it to survive in the environment and it's not susceptible to chlorine or other disinfectants like alcohol. So, because of that, it can survive in these pools, even ones that are properly chlorinated."

Because of its potency, crypto is the single greatest source of waterborne outbreaks among humans in the U.S., the CDC notes.

The parasite can actually live more than a week in chlorinated waters, so the agency recommends "substantial decontamination" for swimming sites with the crypto parasite and details specific guidelines for hyperchlorinating such sites.

Periodic hyperchlorination can also help prevent outbreaks, but the most effective form of prevention likely comes from you. The CDC says to not go swimming if you or your child have recently experienced diarrhea.

"It can spread very easily in the pool and it's very hard to get out of the pool once it's in there," said Michele Hlavsa, chief of CDC’s Healthy Swimming Program. "It's so important to keep it out of the pool in the first place."

<img class="styles__noscript__2rw2y" src="https://www.cdc.gov/mmwr/volumes/68/wr/figures/mm6825a3-F1.gif" srcset="https://www.cdc.gov/mmwr/volumes/68/wr/figures/mm6825a3-F1.gif 400w, https://www.cdc.gov/mmwr/volumes/68/wr/figures/mm6825a3-F1.gif 800w" > Cryptosporidiosis outbreaks by state from 2009-2017. Great Lakes states reported more outbreaks than any other region, but that's likely due to differences in quality of reporting from state-to-state, the CDC noted. (CDC)

Great Lakes states reported more outbreaks than any other region, but that's likely due to differences in quality of reporting from state-to-state, according to Hlavsa and Gharpure. Not every state has the same comprehensive tests for crypto.

Regional differences in temperatures don't do much to account for differences in outbreaks by state because only water temperatures greater than 104 degrees suppress the parasite, Hlavsa said.

Instead, prevention is key, which is why the agency is also trying to more effectively track the origin and spread of these crypto outbreaks.

A goal for the CDC is to get every state on CryptoNet, the first "surveillance system" for the parasite that can also tell the agency the variety of crypto a given place is dealing with, helping to trace its origins and spread.

CryptoNet is already in use in Alabama, Georgia, Maryland, Maine, Minnesota, Nebraska, New Hampshire, Ohio, Oregon, Tennessee and Wisconsin.