You may want to think twice before taking a dip on your next vacation, according to the results of a new report from the Centers for Disease Control and Prevention (CDC).

Between 2000 and 2014, the CDC recorded 493 disease outbreaks related to treated recreational water, resulting in more than 27,000 illnesses and eight deaths. And in almost a third of those outbreaks, the infections could be traced back to hotel pools, hot tubs and spas, the CDC says. In hotels, pools were a major culprit, but 65 cases stemmed from hot tubs or spas.

Of the outbreaks with a confirmed cause, the vast majority — 94% — were due to pathogens, while the remaining outbreaks were caused by chemicals. Cryptosporidium (also called Crypto) — which is a parasite that causes diarrhea and other gastrointestinal issues — caused 212 outbreaks and more than 21,700 illnesses over the 14 years. It’s typically spread when a person swims while they have diarrhea, putting others at risk of swallowing contaminated water. Legionella, which causes the serious pneumonia-like illness Legionnaires’ disease, as well as the less-severe Pontiac fever, was responsible for 57 outbreaks and 624 illnesses. (At least six of the eight deaths were caused by Legionella, the CDC says.) Pseudomonas, which results in swimmer’s ear or a skin condition known as “hot tub rash,” caused 47 outbreaks and 920 infections.

Outbreaks spiked during June, July and August, but they were observed all year long throughout the study period.

The prevalence of waterborne disease outbreaks highlights how difficult they can be to prevent, the report says. Crypto, for example, can survive even in properly maintained and chlorinated pools, and Pseudomonas and Legionella thrive in slimy areas in hot tubs, pools and water playgrounds. While hotels, water parks and public pools can minimize these risks through adequate maintenance, much of the burden of prevention falls on swimmers, especially when it comes to keeping Crypto out of the water.

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Write to Jamie Ducharme at jamie.ducharme@time.com.