Last year saw more than twice as many outbreaks of the parasitic infection known as "Crypto," which is linked to swimming pools and water parks, than were seen two years before that, according to the Centers for Disease Control.
There were at least 32 outbreaks caused by the Cryptosporidium parasite in 2016, up from 16 in 2014, according to preliminary data published in CDC's Morbidity and Mortality Weekly Report, released Thursday.
It can spread through the feces of another person who is infected, including through pool water that's been contaminated with diarrhea, according to the CDC. Chlorine can't easily kill the parasite, meaning it can last for up to 10 days in water that is otherwise properly treated.
One mouthful of contaminated water can leave a healthy person sick with diarrhea, cramps and vomiting for up to three weeks.
"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, chief of CDC's Healthy Swimming program, in a statement. "Protect yourself from getting sick by not swallowing the water in which you swim."
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.
Crypto is the most common cause of diarrhea, illness and outbreaks linked to swimming pools or water playgrounds because it is not easily killed by chlorine and can survive up to 10 days in properly treated water.
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Swallowing just a mouthful of water contaminated with Crypto can make otherwise healthy people sick for up to three weeks with watery diarrhea, stomach cramps, nausea, or vomiting, and can lead to dehydration.
The CDC provides more information here.