An alarming new report from the Centers for Disease Control and Prevention should give anyone who is looking forward to getting in the pool this summer some pause. Authorities have discovered that infections from a nasty parasite that can survive in chlorine-filled swimming pools have doubled in the past couple of years, and experts aren’t quite sure why.

Cryptosporidium, a parasite that causes people to get terrible diarrhea and other symptoms like nausea and cramps, has doubled in terms of outbreaks between 2014 and 2016, going from 16 to 32 outbreaks in a year. The parasite is spread through contact with feces of an infected person.

There were 2,000 people who got sick in Ohio alone, and the increase is notable enough that the CDC has issued a nationwide warning. It’s a tough parasite to deal with, as it can survive normal levels of chlorine in pools, so authorities will need to be proactive to prevent the parasite from getting in pools in the first place.

“At least 32 outbreaks caused by Cryptosporidium (also known as “Crypto”) linked to swimming pools or water playgrounds in the United States were reported in 2016, compared with 16 outbreaks in 2014, according to preliminary data published today in CDC’s Morbidity and Mortality Weekly Report,” the CDC statement reads. “The parasite can spread when people swallow something that has come into contact with the feces (poop) of a sick person, such as pool water contaminated with diarrhea.

“In comparison, 20 Crypto outbreaks linked to swimming were reported in 2011, 16 in 2012, and 13 in 2013. It is not clear whether the number of outbreaks has increased or whether better surveillance and laboratory methods are leading to better outbreak detection.”