Crypto is the most common cause of diarrheal 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. 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.
source: CDC Website