BOISE, Idaho — Before you jump into the pool to cool off, the CDC is warning swimmers of a chlorine-resistant parasite that's been causing outbreaks across the country.

Cryptosporidium or "crypto" is commonly spread at public pools and can cause diarrhea that lasts for weeks.

Crypto has increased by 13 percent each year since 2009 and is spread through fecal matter, typically after someone swallows the parasite in contaminated water or food.

Paula Lawson, Boise's aquatic program manager, says in 2011, a major crypto outbreak shut down dozens of pools in both Idaho and Utah.

"They connected that it was probably people traveling back and forth, which caused all the pools to shut down early," Lawson said. "It just transfers very easily from swimmers going to different pools, maybe potentially a sick swimmer going to different pools."

To prevent another major outbreak, all Boise public pools have installed a UV system, which kills parasites like crypto.

"The circulated water passes through the UV system and that kills like 99.9 percent of all bacteria or any types of growths that could be present in the water," Lawson said.

Roaring Springs, the largest waterpark in the Northwest, has followed suit, installing a massive UV system this past spring.

"With our new ultraviolet light system, we can confidently say we will send all of those guests home without any waterborne illness," said Tiffany Quilici, the marketing director for Roaring Springs. "Water is our business at Roaring Springs so it was definitely worth the $350,000 investment in our UV system."

While the equipment certainly isn't cheap, both Qulici and Lawson say it’s worth it, keeping thousands of guests safe.