(CNN) -- The Centers for Disease Control and Prevention are warning people before you head to the pool to shower after swimming.
There could be a parasite in the water that could literally turn your stomach.
It's called cryptosporidium - or "crypto" for short.
The parasite is 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.
The CDC says 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.
Chlorine and filters aren’t enough to keep crypto away, so if you go to the pool, shower after your swim.
And if you know anyone who has had diarrhea recently, don’t let them in the pool.