While you’re cooling down this summer in the communal bathtub known as your local swimming pool, a federal agency wants you to keep one rule in mind: Don’t drink the water.

After all, it may not be just water that you’re swallowing.

That’s the message from the Centers for Disease Control and Prevention, which in a recent report warned swimmers of the steady rise of a microscopic fecal parasite that can lurk for days in pools and splash parks, and cause serious intestinal problems.

One of the most common ways the waterborne parasite is transmitted is through swallowing contaminated water in pools or water playgrounds, the report said.

According to the researchers, the number of reported outbreaks of illness caused by the single-cell parasite Cryptosporidium — “Crypto” for short — has been rising steadily since 2009, spiking annually in the summer months.

Between 2009 and 2017, 7,465 cases of cryptosporidiosis — which can cause watery diarrhea for up to three weeks — were recorded in 40 U.S. states and Puerto Rico, with nearly half of the cases occurring in eight Great Lakes states, including Pennsylvania. The number of reported outbreaks has risen an average of 13 percent each year.

The Philadelphia Health Department did not respond to a request for comment on whether any cases had been recorded in the city. Philadelphia Parks and Recreation officials, meanwhile, said they were taking steps to keep pools clean.

The CDC notes that Crypto is one pool-loving parasite. Protected by an outer shell, the bacteria is hard to kill and can survive outside the body for long periods of time, unfazed by chlorinated water.

However, a little hygiene at the pool goes a long way, the agency said.

While a quarter of Americans say they would swim with diarrhea, do not get in the pool if you’re sick or allow ill children to swim, the CDC recommended.

“Young children can get seriously sick and easily spread" the condition, said Michele Hlavsa, chief of CDC’s Healthy Swimming Program. "They don’t know how to use the toilet and wash their hands or are just learning how. But we as parents can take steps to help keep our kids healthy in the water, around animals and in child care.”

And if you’re diagnosed with cryptosporidiosis, which has other symptoms including nausea, vomiting, and stomach cramps, refrain from taking a dip until two weeks after the diarrhea completely stops, the CDC said.

At Philadelphia’s pools, the Parks and Recreation Department said it follows Pennsylvania code when it comes to cleaning, including running chlorinators 24/7, testing chlorine levels hourly, treating pools after fecal accidents according to CDC guidelines, and not allowing young children with regular, non-swim diapers in the pool.

“The Health Department inspects all public pools throughout the city, including those in Philadelphia parks and recreation centers prior to opening for the season, and will not allow them to open if they are not satisfied with the cleanliness and safety of the pool and pool area,” the department said in a statement.

Anyone with concerns about unsafe water in Philly pools can call the Department of Public Health at 215-685-7342.