Poop in the pool is making people sick, says the Centers for Disease Control and Prevention.
Cryptosporidium outbreaks linked to swimming pools and water playgrounds have doubled since 2014.
Crypto is the most common cause of diarrheal illnesses and outbreaks linked to swimming pools and water parks.
Swallowing just a mouthful of contaminated water can make you sick for up to three 3 weeks.
It is not easily killed by chlorine, and can survive up to 10 days in properly treated pools.
Infected people may suffer diarrhea, stomach cramps, nausea, vomiting and dehydration.
For people with weakened immune systems, symptoms can be severe and could lead to severe or life-threatening illness.
"To help protect your family and friends from Crypto and other diarrhea-causing germs, do not swim or let your kids swim if sick with diarrhea,” said Michele Hlavsa, R.N., M.P.H., chief of CDC’s Healthy Swimming Program.
“Protect yourself from getting sick by not swallowing the water in which you swim.”
There were 32 reported outbreaks in 2016 in the U.S.