Every summer, the Centers for Disease Control and Prevention turns its attention to the disgusting state of American swimming holes — and their latest investigation traced thousands of infections back to lakes, rivers, and the ocean.

A team of researchers analyzed 140 outbreaks that made nearly 5,000 people sick, and even killed two swimmers between 2000 and 2014. Public parks and beaches accounted for roughly two-thirds of the outbreaks, according to the CDC’s latest Morbidity and Mortality Weekly Report. And the majority of those outbreaks occurred over the summer months — right when the water is most inviting.

The worms can also wiggle into human skin and cause a rash

The most common infections the CDC turned up came from swallowing poop-tainted water. Most of them cause diarrhea — norovirus, bacterial infections like Shigella and E. coli, and parasites like Cryptosporidium were all reported. People also got itchy rashes thanks to parasites called avian schistosomes, which usually infect birds and a specific species of snail. But the worms can also wiggle into human skin and cause a rash before the misguided parasites die. And toxic algal blooms sickened swimmers in a handful of outbreaks.

It’s not all diarrhea and rashes; two people died from infections with a brain eating amoeba called Naegleria fowleri. To avoid it, the study authors say, use a nose clip to keep your nostrils shut and don’t dunk your head underwater while swimming in warm freshwater and hot springs.

It’s enough to make you want to stay out of the water

It’s enough to make you want to stay out of the water entirely, especially since it’s not just lakes, rivers, and oceans that can be cesspools. A CDC report published in May traced nearly 500 outbreaks that sickened more than 27,200 people to pools, spas, and waterparks. A third of those were located at hotels, motels, inns, and lodges. Major culprits included bacteria that cause a dangerous pneumonia called Legionnaires disease; Pseudomonas, which causes a rash and swimmer’s ear; and, again, the diarrhea-dealing parasite, Cryptosporidium.

Most of these infections thrive in dirty water (although Crypto can survive even in chlorinated pools). And that’s especially alarming given that in 2016, the CDC reported health or safety violations at 80 percent of public aquatic facilities in five different states. Most were because of inadequate disinfection, and the worst violations were in kiddie pools. Gross.

Of course, disinfection can cause problems of its own, according to yet another CDC report. This one investigated strange symptoms at an indoor water park in Ohio, where employees reported trouble breathing, burning eyes, and vomiting. The CDC eventually concluded that chlorine disinfectants were reacting with proteins from swimmers’ pee, sweat, dead skin, and lotions. The chemical reaction combined with a broken ventilation system and created a noxious cloud that employees then inhaled.

If the water looks gross, don’t swim in it

So what’s a swimmer to do? First off, pay attention to signs, this latest CDC report says. If a sign says don’t swim, then don’t. swim. Also if you have diarrhea or have recently had diarrhea, please don’t lower your infectious body into a body of water shared with other people. Of course, outdoors in lakes, ponds, rivers, and the ocean, poop can get into the water even without someone actively pooping in it. For instance, stormwater runoff can flush human and animal feces into the water. So can flooding, discharges from sewage treatment plants, and boats.

A good rule of thumb is if the water looks gross, don’t swim in it — particularly if there are dead things like fish in it already, the study says. And even if it looks pristine, don’t swallow it, and definitely don’t snort it. Happy swimming!