Remember when summer meant splashing willy-nilly in some public pool, your mom yelling from the side, "Don't swallow the water!"

The Centers for Disease Control and Prevention would like you to call your mother, and tell her she was right.

The CDC recently released a report that shows a rise of 13% per year in tracked outbreaks of illness caused by the single-cell parasite Cryptosporidium, the leading cause of waterborne disease outbreaks in the United States. The outbreaks, which spike annually in July and August, are often attributable to public pools.

That's partly because, according to Dr. Radhika Gharpure of the CDC's Waterborne Disease Prevention Branch, Crypto parasites, which are spread through the poop of infected humans or animals, feel right at home in swimming pools. "Unlike other parasites or bacteria," Gharpure says, "Crypto is very tolerant to chlorine and can survive in a properly chlorinated pool for several days — so once it gets into the pool, it can be hard to get out."

Those pool chemicals — even the ones strong enough to fade your swimsuit — aren't helping with the parasite problem.

Crypto infections can come from other sources, too, such as livestock, or contaminated food or drinking water. A 1993 outbreak in Milwaukee, which remains the largest outbreak of waterborne illness in the U.S., sickened more than 400,000 people and was caused by improperly filtered drinking water. City residents were forced to boil water for a full week, drugstores sold out of anti-diarrhea medications, and 69 people (93 percent of whom were immunocompromised AIDS patients) died.

Still, Crypto is "definitely associated with swimming pools and water parks," says Dr. John Flaherty, a professor of medicine at Northwestern University Feinberg School of Medicine who's an expert in infectious diseases. "That was really brought home to me a few years ago when I was staying at a place with a water park and there was a sign that said, 'If you have Cryptosporidiosis, don't use the pool.' I mean, I doubt most people would even know what Cryptosporidiosis is." Most people who become infected, Flaherty says, experience watery diarrhea for several days and think they have a case of food poisoning or a stomach flu. Many are never diagnosed, and part of the reported rise in cases is attributable to recent improvements in the tracking of outbreaks.

That lack of awareness around Crypto is a big part of the swimming pool problem. In a recent survey exploring Americans' swimming pool habits, commissioned by the Water Quality and Health Council, 24% of respondents said they would go swimming in a pool within one hour of having diarrhea, and 48% reported that they never shower before swimming, essentially turning the pool into a huge public bathtub.

"I mean, c'mon people, do us a favor," says Flaherty, who points out that, while Crypto is "not going to leak out of you microscopically" it could be present on your skin if you are having diarrhea, and a child in diapers who is having diarrhea could definitely contaminate the pool water. "It's more common in kids," he says, "so if your kid is having diarrhea don't bring them to the pool. And if you're having diarrhea, don't go to the pool, because it is a risk."

"Crypto germs in poop can immediately cause infection," says Gharpure. "It only takes a few Crypto germs to make someone sick and there can be millions of Crypto germs in poop so it's critically important to keep Crypto out of the pool."

Melaney Arnold, spokesperson for the Illinois Department of Public Health, which requires that pools connected to an outbreak be closed so that the entire facility can undergo cleaning and water samples can be examined by the IDPH, points out that the presence of Crypto "is not an indicator of water quality or operational standards of the facility" but is more likely to be "carried into the pool by someone."

In other words, it's yet another public health issue in which we are totally dependent on the good hygiene of our neighbors. And if you've been in a public restroom lately, you might have reason to doubt that your health is in good (and recently washed) hands. That doesn't mean you have to swear off swimming, though.

Luckily, illness caused by Crypto isn't typically something to be alarmed about, unless you are immunocompromised due to a transplant or other underlying condition or illness, or you are pregnant. "In most other cases, like many things we get, it's a nuisance," says Flaherty.

And, he says, jumping into that public pool is probably an acceptable risk. "I cross the street, I eat raw oysters occasionally, and I will probably be going to the Lincolnwood pool tonight with my daughter and grandchildren," he says.

But you can bet he won't be swallowing any pool water.