Memorial Day weekend is here along with the unofficial start of summer and pool season. >> Read more trending news But, before taking those refreshing dives that help make the hot, sultry weather more bearable, the Centers for Disease Control and Prevention offered this warning: That sparkling blue water may not be as clean as it appears. Sometimes lurking in those inviting ripples are all manner of uninvited guests, like parasites and bacteria and a host of other foulness. And they’re not just hanging out in backyard and community pools. They show up in hot tubs, water parks and splash pads, too. Just over 20 years ago, 26 children were sickened by a strain of E. coli bacteria after playing in the kiddie pool at White Water Park in Marietta, Georgia. Seven of the children were hospitalized; one died. A state investigation determined the chlorine level was too low in the pool, a finding that helped raise awareness about the importance of proper monitoring and ushered in a new era of vigilant oversight in Georgia. >> Trending: Here’s one way to prevent weight gain during holidays such as Memorial Day Nationally, from 2000 to 2014, close to 500 outbreaks of waterborne illnesses were reported in recreational venues in 46 states and Puerto Rico, causing 27,219 cases of sickness and eight deaths, according to the CDC. Pool chemicals can take care of many problems, but not always. Getting optimal results for safety comes down to having the right balance. And, to be frank, it comes down to having the right users — those who adhere to, let’s just say, proper pool etiquette. “We want people to go to pools and have a happy, good time,” said Michele Hlavsa, chief of the CDC’s Healthy Swimming Program. “We just want to make sure people are doing it in a safe, healthy way.” To get down to the nitty gritty, most of the illnesses in pools, hot tubs and water playgrounds are caused by Cryptosporidium -- that’s Crypto, for short. Crypto is a parasite that causes Cryptosporidiosis, which leads to diarrhea. >> Related: Giant, 10-million-pound boulders, one as large as house, shut down Colorado highway Crypto — brace yourself — is spread by swallowing water that has been contaminated with fecal matter. Most germs are killed within minutes by common pool disinfectants like chlorine or bromine, but Crypto is a germ that can survive in properly chlorinated water for more than seven days. The diarrhea it causes can last for up to three weeks. And the number of Crypto cases have been steadily rising, with twice as many in 2016 as in 2014. Hlavsa said even a single mouthful of contaminated water can lead to a Crypto illness. She recommends parents encourage their children to not swallow water when swimming. And there are other steps you should take for healthy swimming. Hlavsa recommends that families check a pool’s inspections online or on site. In Georgia, public and semi-public pools (such as pools at apartment complexes), as well as hot tubs and water parks, are inspected by local health departments. Backyard pools are not. >> Trending: Marvel Comics legend Stan Lee allegedly suffered elder abuse; former manager arrested She also recommends that families check to make sure the drain at the bottom of the deep end of a pool is clearly visible – a sign that the water is crystal clear. Other safety measures should include making sure drain covers at the bottom of the pool appear to be secured and in good shape. Even if you’re not spreading Crypto, people need to consider everything on their bodies — sweat and dirt, dust, deodorant and hair spray — before jumping into communal waters, Hlavsa said. A recent survey found that more than half of Americans (51 percent) use a swimming pool as a bathtub — either swimming as a substitute for showering or using the pool to rinse off after exercise or yard work. It’s a habit that’s taken hold, even though nearly two-thirds of Americans report they know chemicals do not eliminate the need to shower before swimming. And that’s not all. The survey — commissioned by the Water Quality & Health Council, a group of advisers to the chlorine industry trade association — found 40 percent of Americans readily admitted they’ve urinated in a pool as an adult. While it may seem obvious that this is not a good idea, Hlavsa explained the science of why it’s not: Urine reacts with chlorine, reducing the amount of chlorine available to kill germs. That bodily fluid, along with dirt and sweat, mixed with chlorine creates chemicals called chloramines, which causes red and itchy eyes. Bottom line: It’s not the chlorine alone causing eyes to redden and sting. >> Trending: Detroit teen shot, killed for pricey Cartier glasses; police search for suspects And speaking of chlorine, people can check water quality themselves by purchasing pool test strips at hardware stores that measure the level of the chemical and the pH level, Hlavsa said. She uses a test strip to check on water quality before swimming with her young children, who are 2 and 4. Chlorine levels should be at least 1 part per million for pools and at least 3 parts per million in hot tubs, according to the CDC. Getting the mix right isn’t always easy. Too little means germs can grow and spread. Too much chlorine can irritate the skin and eyes. Read more here.