Every summer, warm weather sends people across the country looking for a cool refuge in public pools, hotel pools, spas, and other water-based destinations. Before you take the plunge, you may want to heed the advice of the Centers for Disease Control and Prevention (CDC). Jumping into a publicly-populated pool could be like bathing in someone else’s diarrhea, as Men’s Health reports.

The health agency revealed its findings in their Mortality and Morbidity Report, which explains why pools are ground zero for bacteria. Between 2000 and 2014, the CDC traced 493 outbreaks and over 27,000 cases of illness that could be connected to exposure to a public pool. The primary culprit was Cryptosporidium, a parasite found in feces that causes intestinal distress. The determined little bugs can survive for up to seven days after encountering the CDC’s recommended levels of one to three parts per million (PPM) of free chlorine. Even if the pool is being cleaned and maintained properly, Cryptosporidium can idle long enough to infect someone else. The report also indicated that Legionella (which causes Legionnaire’s disease) and Pseudomonas (responsible for ear infections and folliculitis) were found in some of the pools.

The problem is likely the result of swimmers entering the pool while suffering from an upset stomach and leaving trace fecal matter behind. The CDC recommends that you not enter a public pool if you feel unwell, that you ask for a pool inspection report if you’re concerned about the hygiene of the facility, and that you absolutely not swallow any water. The agency also recommends that any pool owner who has experienced a “diarrheal incident” in their water opt for hyperchlorination to kill bacteria.

[h/t Men’s Health]