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 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.

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