The Centers for Disease Control is warning pool-goers to be wary of certain kinds of bacteria that could be in the water.
One danger, in particular, a parasite called cryptosporidium or "crypto" for short. It's spread by feces and the CDC says they're seeing more and more people come down with it.
In a report that came out Friday, the CDC says they've seen a nearly 13 percent increase every year between 2009 and 2017.
Most people who get crypto get it from the pool, but you can also get it from lakes and cattle.
They say it can cause "profuse, watery diarrhea" for as long as three weeks. It can be worse for kids, pregnant women and anyone who has a compromised immune system.
The CDC says most germs and bacteria are killed by common pool disinfectants like chlorine and bromine, but crypto could survive in those conditions for more than seven days.
The agency recommends not letting anyone swimming if they are sick with diarrhea, shower for at least one minute before getting in the water or even inspecting the pool yourself. Once in the water, be sure not to swallow any of it and be sure to change any diapers away from the poolside.