Somewhere in America this summer, a small child will have a diarrhea incident in a pool. The kid will freeze, waiting for that embarrassing blob of shame to envelop them, forever labeling them a pool-pooper. But sometimes, that blob might never come. The kid will keep swimming merrily, not knowing they’ve accidentally released millions of parasites into the pool. Some of those parasites, say scientists with the CDC, can survive even the harshest chlorine.

This parasite, called Cryptosporidium (Crypto for short), usually lives in the human gut. But if you come into contact with it in the outside world — i.e. if someone has an accident in the pool and doesn’t own up to it — it could make you incredibly sick.

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In May, a CDC report showed that Crypto is responsible for almost 89 percent of illnesses borne from recreational swimming. What CDC researcher Michele Hlavsa of the National Center for Emerging and Zoonotic Infectious Diseases tells Inverse about Crypto is very gross and counterintuitive, but it is very important to know when it comes to pool safety. Sometimes, in cases of very watery diarrhea, you may never know that someone had an accident in the pool.

“So what happens then is that the Cryptosporidium just sits in there. No one knows there’s diarrhea, and the embarrassed swimmer doesn’t say anything,” Hlavsa tells Inverse. “That Crypto can live in that pool for seven or more days.”

This happens, Hlavsa says, because Crypto is a unique type of parasite encased in a hard outer shell, which makes it almost impossible for a typical cleaning agent, like chlorine, to penetrate. The only way to pierce Crypto’s biological body armor is to blast the pool with levels of chlorine that are incredibly high. This, Hlavsa says, usually takes days of intervention, during which swimmers become “antsy,” and pay-to-swim public pools tend to lose money.

Crypto outbreaks have quietly but steadily infected America’s pool-goers over many years, and the problem seems to be getting worse. Between July and October of 2016, the CDC reported that Arizona had identified 352 individuals who became sick because of Crypto parasites. Between 2011 and 2015, they had only identified 62 cases. Ohio had even more startling results: In 2016, it reported 1,940 infected individuals. Outbreaks like this happen because Crypto moves easily from pool to pool with the swimmer and, because of chlorine resistance, persists long after patient zero has moved on to a different pool party.

“If I as a swimmer have a diarrheal incident in one pool, that pool is then shut down and treated and whatever. Then I got to another pool and contaminate that pool. I have the potential of getting the whole community pretty sick pretty quickly,” Hlavsa says.

The CDC report released in May found that infections tend to spike in the summer, when most people are swimming. 

The CDC’s Weapon: “CryptoNet”

Controlling Crypto outbreaks is hard, partially because of its hard outer shell, and partially because it requires owning up to pooping in the pool. But from a scientific standpoint, we have only recently begun to understand it.

As recently as 1990, the general scientific consensus was that there was only one species of Crypto that could infect humans, called C. parvum. But recent advances in genotyping technology have revealed that there are actually 30 different subspecies of crypto parasites. Several of these species are capable of infecting humans with different levels of severity, and each is specific to a particular environment. For instance, the C. hominis parasite tends to flourish in urban environments, whereas the C. parvus parasite is usually seen in rural areas because it can infect both humans and pre-weaned calves.

Cryptosporidium parvum, common in rural areas

But there is one distinguishing challenge facing experts studying Crypto: All the species of this nasty pool-borne parasite look exactly the same. One CDC fact sheet reports that “only molecular methods can distinguish these species, genotypes, and subtypes.” In short, you’d have to do a full-blown DNA test to figure out which type of Crypto you’re dealing with. Armed with this knowledge, the CDC has actually launched a molecular tracking system intended to investigate the different members of the Crypto family. The researchers behind CryptoNet, who released the first comprehensive study in 2017, have made significant strides in illuminating which species of Crypto can survive longer than others.

While this research is promising, Hlavsa is adamant that the best way to prevent the spread of Crypto doesn’t require lab science. She suggests to not swim for two weeks if you’ve had a diarrheal incident, be sure to take kids on regular bathroom breaks, and check publicly posted pool inspection scores. In fact, the CDC is set to release a new set of aquatic health code guidelines in late July to help improve pool safety across the country.

In the meantime, if you poop in the pool, do us all a favor and just own up to it.