As the weather warms, schools let out, and people head to pools and water parks, the Centers for Disease Control and Prevention wants to once again warn you of the dangers lurking in those cool, chlorinated waters.

This year, the agency is drawing attention to an uptick in pool-associated outbreaks of Cryptosporidium, aka Crypto. The protozoan parasite is spread by the stool of sick swimmers. A single “fecal release” can unleash tens of millions of hardy oocysts, which can survive in properly chlorinated pool water for up to 10 days. If just a few of the tiny critters slips into a swimmer's mouth or nose, they can cause stomach cramps, vomiting, and diarrhea.

You might not think that fecal matter is very common in pool water... oh, but it is. As the CDC pointed out in another pool warning from May of 2013, 58 percent of public pools tested positive for fecal bacteria. (A recent Canadian study suggested that large public pools contain an average of about 75 liters of urine , too.)

Last year, there were 32 reports of Crypto outbreaks at pools in the US. There were only 16 reported in 2014, the agency notes in its latest May announcement. In the past decade, outbreak reports have mostly hovered between 15 and 20. But in 2007 there were 40, and in 2009 there were fewer than 10.

The CDC recommends that people should not swim if they’re sick, nor should they let sick kids swim. In fact, people recovering from a Crypto infection should wait two weeks before swimming again. Healthy swimmers should avoid swallowing any water and rinse off after each dip, the agency says.

The only way to get rid of Crypto in a pool is to shut it down and hyper-chlorinate it. But that action also has risks, as the CDC warned in May of 2014 and May of 2009. Injuries from pool chemicals leads to around 5,000 emergency room visits each year, the agency reported. Pool operators should carefully follow directions on pool chemical product labels, secure chemicals, and keep them away from kids.

Overall, the agency has repeatedly found that pool operators are not good at maintaining pools. In a May warning from 2016, the CDC reported that nearly 80 percent of public pool inspections uncovered at least one health and safety violation. One in eight pools had to be immediately closed. The most common violations include improper water pH and disinfectant levels. The agency reported similarly alarming data on pool inspections in May of 2010.