With summer almost upon us, health officials are warning Americans to take precaution against nasty germs that could be lurking in swimming pools. Waterborne disease outbreaks from treated recreational water such as pools, hot tubs and water playgrounds have been common in recent years -- and it turns out hotel swimming pools are among the biggest culprits.

A new report from the Centers for Disease Control and Prevention (CDC) finds that a third of the outbreaks over a 14-year period occurred at hotel swimming pools.

The report, published in CDC's Morbidity and Mortality Weekly Report, looked at data from 2000 through 2014 and found that 493 outbreaks were reported, resulting in at least 27,219 illnesses and eight deaths.

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Cryptosporidium, also known as "Crypto," a parasite tough enough to survive even in properly maintained pools, was the most common cause of illness. Crypto was responsible for 58 percent of outbreaks, and 89 percent of all illnesses, where a germ was identified linked to pools, hot tubs and water playgrounds.

Crypto spreads in pool water when someone who is sick with the parasite goes swimming and has diarrhea in the water and then others swallow the contaminated water. Parents of young children play a key role in preventing Crypto outbreaks.

"Swallowing just a mouthful of water with Crypto in it can make otherwise healthy kids and adults sick for weeks with watery diarrhea, stomach cramps, nausea, and vomiting," Michele Hlavsa, chief of CDC's Healthy Swimming Program, said in a statement. "Chlorine cannot kill Crypto quickly. We need to keep it out of the water in the first place. Don't go into the water, and don't let your kids go into the water, if sick with diarrhea."

Other causes of waterborne disease include the bacteria Legionella -- the source of Legionaires' disease -- and Pseudomonas. Legionella can cause severe pneumonia and symptoms similar to the flu and accounts for 16 percent of outbreaks during the study period. Pseudomonas can lead to a skin rash and ear infection and made up 13 percent of the outbreaks.

Both bacteria can survive disinfectants in slimy areas called biofilm. It's harder to kill Legionella and Pseudomonas when they're protected by biofilm, so pool operators need to maintain proper cleaning practices and disinfectant levels to prevent these bacteria from growing and causing illnesses in swimmers.

Certain people are more likely to get sick from Legionella, including adults 50 years or older, current or former smokers, people with chronic lung disease, and people with a weakened immune system. Individuals who fall into these categories should see their doctor immediately if they develop symptoms and could have been exposed to Legionella in a hot tub or pool.

To help stay safe in the pool, the CDC recommends:

Don't swim or let your kids swim if sick with diarrhea. If Crypto is the cause of the diarrhea, wait until 2 weeks after diarrhea has stopped to go swimming.

Don't swallow pool water.

Take kids on bathroom breaks hourly, and change diapers in a diaper-changing area away from the water.

Before swimming, check the facility's inspection score.