Find out about the germs that lurk beneath the surface at swimming holes — and what you can do to protect yourself and your family.

Nothing’s more refreshing than a dip in the pool or a nearby lake. But gulp! Even a mouthful of contaminated water can put a serious damper on your summer fun.

“Pools and untreated water from lakes and other sources can contain all kinds of environmental organisms that could make you sick,” says Benjamin Galvan, infection preventionist at Tampa General Hospital in Florida and spokesperson for the Association for Professionals in Infection Control and Epidemiology. Don’t let water-related illnesses dampen your outdoor adventures. Here’s a breakdown of the most common germs in bodies of water — and what you can do to stay safe.

Giardia, a tiny parasite that causes diarrhea potentially lasting two weeks or more, tops Galvan’s list. The parasite can leach into fresh water from wildlife feces. After all, “animals have no reason not to defecate in the water,” Galvan says. Giardia can also infect people in pools or water playgrounds. If one person infected with giardia has diarrhea or accidentally poops in the water, the water can become contaminated with tens of millions of giardia germs, according to the Centers for Disease Control and Prevention (CDC). If you swallow the contaminated water, you could get infected.

Giardia has a tough outer shell, which allows it to survive up to 45 minutes, even in properly chlorinated pools and water playgrounds. “Chlorinated pools continuously kill organisms, but swimmers could get sick if the right cleaning measures aren’t taken after a fecal contamination event. For example, after the pool or water park has been evacuated, some guidelines recommend increasing the water chlorination level and making sure all filtration systems are working properly,” Galvan says. Swimming in a pool that’s not properly chlorinated, or swimming too soon after a fellow swimmer has befouled the water, can also lead to illness. And, of course, at public pools and water parks, there may be fecal accidents that sneak under everyone’s radar.

Cryptosporidium, or crypto, is another common water contaminant that could make you sick. From 2015 to 2019, the CDC reported 208 U.S. outbreaks associated with recreational water, including public pools, hot tubs, and water playgrounds, which resulted in 3,646 cases of illness, 286 hospitalizations, and 13 deaths. Of the 75 percent of the outbreaks with a confirmed diagnosis, nearly half were caused by crypto, which causes gastrointestinal symptoms such as diarrhea. Crypto, which is highly resistant to chlorine, can take more than seven days to kill, Galvan says.

That same report found that more than 40 percent of the reported and diagnosed illnesses were caused by legionella, which causes Legionnaires’ disease, a severe pneumonia, and Pontiac fever, a milder illness with flu-like symptoms.

Other potential water contaminants that can leach from human or animal feces and make you sick include shigella, norovirus, and E. coli.

In rare cases, waterborne illness can be caused by Naegleria fowleri, a brain-eating amoeba that can travel to the brain from recreational water that enters the nose. The brain-eating pathogen may be found in warm, freshwater lakes and rivers, mostly in the Southern U.S. states. Fortunately, illness from Naegleria fowleri is rare. From 2012 to 2021, only 31 infections were reported in the United States, according to the CDC. (Still, the disease is often fatal.)

Be on the lookout for these common symptoms of waterborne illness, which can show up within a day or so of swimming or spending time in pools and lakes.

“For mild cases of diarrhea and vomiting, replace lost fluids with water, Pedialyte, Gatorade, or popsicles,” says Riza Taddy Conroy, MD, associate clinical professor of family medicine at The Ohio State University Wexner Medical Center.

See your doctor if you have severe diarrhea — four or more runny or watery bowel movements in one day — severe stomach pain, a tender rash, or any of the above symptoms that last for more than two weeks. If you have diarrhea, your doctor can do a stool culture to determine if bacteria is the cause and prescribe the appropriate antibiotic, if necessary.

Go to the hospital emergency department immediately if you or a family member has symptoms of Naegleria fowleri, the brain-eating amoeba, which can include fever, nausea, severe headache, vomiting, a stiff neck, and seizures.

Swimming is healthy and fun. Still, it pays to play it safe. Here’s what you can do to reduce your risk of getting a waterborne illness:

Keep your mouth shut. “Don’t ingest any pool water, or untreated water in general, because you don’t know what’s in it,” Galvan says. The best strategy is to not open your mouth. It’s especially important to keep pool or lake water out if you have a weakened immune system; older people, pregnant women, young children, and people with cancer or HIV fall into that category. “If you do happen to swallow pool or lake water, spit it out and rinse your mouth as soon as possible,” Dr. Conroy says.

Shower before getting into a public pool. The CDC reports that people typically have about 0.14 grams of poop on their body at any given time. That poop can wash off of the swimmer’s body and contaminate the water with germs. That’s why it’s a good idea to shower before entering a pool or water park.

Stay home if you’re sick. If you or your kids have an active case of diarrhea, don’t go in the water.

Check public health advisories. Before heading to a lake or another body of untreated water, check the local public health department’s website. “If there’s a known issue, public health will be aware of it, and they’ll make sure everyone will know about it,” Galvan says.

Avoid swinging off of trees or jumping into lakes and watering holes. “Those types of activities will increase your risk of potentially ingesting or having water forcibly pushed up your nose or into your mouth,” Galvan says. If you can’t resist jumping into the lake or splashing around in it, consider wearing nose plugs.

Test the water. It may seem odd, but you can check the water at the community pool before going in. Most superstores, hardware stores, and pool supply stores sell test strips. “The CDC actually recommends buying testing strips you can put in the water at public pools to see if it’s actually at the appropriate level of chlorination,” Galvan says. Testing strips will indicate the free chlorine level (amount available to kill germs), which should be at least 1 part per million (ppm) in pools and water playgrounds, and at least 3 ppm in hot tubs. The level of bromine (a disinfectant similar to chlorine) should be at least 3 ppm in pools and water parks and at least 4 ppm in hot tubs. The pH, which affects how germs are killed, should be 7.2 to 7.8.

Take frequent bathroom breaks. At pools and water parks, take your kids on bathroom breaks or check their diaper every 60 minutes or so. “Even if your child is wearing swim diapers, check it every hour, away from the pool,” says Cindy Prins, PhD, MPH, clinical associate professor of epidemiology at the University of Florida in Gainesville. “Swim diapers aren’t a long-term solution for containing fecal matter.”

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