The Centers for Disease Control and Prevention have issued an urgent warning about public pools after coming to the alarming discovery that infections from a nasty parasite have surged in recent years, doubling between 2014 and 2016 in the United States. Cryptosporidium, which is a germ that can cause people to become sick with diarrhea for weeks, is a parasite that spreads through contact with the feces of someone who is infected.

CDC reported 32 outbreaks from swimming pools or water playgrounds nationwide in 2016, compared to just 16 in 2014. A total of 2,000 people got sick just in Ohio. Other symptoms besides diarrhea include stomach cramps, nausea, and vomiting, which can lead to severe dehydration.

The problem is the crypto is very difficult to kill. It can’t be dispatched with normal levels of chlorine, so it is paramount that authorities figure out ways to keep it from getting into the pool int he first place. Fortunately, people who get the disease will get better without treatment most of the time, but it can lead to serious complications and it’s definitely not pleasant to get even if you heal from it.

“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,” CDC said in a statement. “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.”