Splashing around in a lake, ocean, pool or water park can be “good, clean fun”—not to mention healthy exercise. The fun factor evaporates, however, if the water you’re frolicking in is teaming with germs that can make you sick. The good news is that there’s no reason to curtail aquatic activities…as long as you take some precautions.

According to a recent University of Illinois study, 90 million cases of stomach, respiratory, ear, eye and skin-related illnesses linked to swimming and other water activities in US lakes, ponds, rivers and oceans are reported yearly. Diarrhea is the most common symptom of a waterborne illness, which usually clears within a few days. Diarrhea or other symptoms that last longer than that should be checked by a doctor. Some recreational water-related illnesses can be serious—even life-threatening—especially for infants and people with compromised immune systems.

Whatever kind of recreational water you dunk in, taking off your wet clothes and washing your whole body (and hair) as soon as possible can go a long way toward keeping you healthy. Here are some other precautions to take…

Pools, Water Parks

Cryptosporidium, a tiny parasite that causes the intestinal infection cryptosporidiosis or “crypto” is the main germ threat from public pools, water parks and similar spots. According to the most current data from the Centers for Disease Control and Prevention (CDC), outbreaks of crypto in public pools and water playgrounds are increasing—up an average of 13% each year during 2009 to 2017.

The parasite is passed in fecal matter when an infected person—most often, young children and babies wearing swimming diapers—has diarrhea in pool water and then others accidentally ingest the water. Even a tiny amount of contaminated water can make you sick. Symptoms include bloating, cramping and watery diarrhea, and last about up to three weeks. Most otherwise-healthy people recover on their own—although it’s important to drink enough fluids to avoid dehydration. People with compromised immune systems are at risk for serious complications.

Chlorine and other chemicals typically used for disinfection in pools can’t completely kill the parasite, so even properly maintained pools can pose a risk. Anyone who has diarrhea should avoid swimming in public water until two weeks after the diarrhea has completely stopped. And whether you have diarrhea or not, showering before you enter the water can help cut down on spreading many other kinds of illnesses as well. But you can’t control what others do (or let their children do).

Best defense: Before using a public pool or water playground, such as at a hotel, water park or apartment complex, check for the latest inspection score on the state or local health department website. The score may also be posted on-site. The CDC also recommends doing your own mini-inspection.

Oceans

Feces—human and animal—can contaminate natural water in lakes, ponds, rivers and oceans. The Environmental Protection Agency (EPA) [[fact checker: please confirm. One source says they do and the other is not sure.]] and other groups, such as the National Oceanic and Atmospheric Administration (NOAA) and the Lake Pontchartrain Basin Foundation (LPBF) in Louisiana, routinely test oceans, lakes and other recreational natural waters for the presence of Enterococci bacteria. Enterococci serve as a marker—high levels indicate the presence of human or animal fecal bacteria in the water. The risk is especially high after rain, when runoff—including from farms where there are livestock—washes feces and germs into waterways.

Bacteria, viruses and parasites in feces can cause upset stomach or diarrhea if the water is swallowed. Other germs in contaminated water can cause skin infection. Wounds, including from surgery or fresh body piercings, are especially vulnerable to infection.

Best defense: It may seem obvious, but if a beach is closed or has an advisory posted for high bacterial levels or other conditions, such as sewage spills—don’t go in the water.

You can also check water quality, beach closings and advisories, sources of possible pollution and other beach safety information on the EPA’s Beach Advisory and Closing Online Notification (BEACON) site, which has a clickable map of US coastal and Great Lakes beaches. Or check the free online Swim Guide available from Swim Drink Fish, a Canadian consortium of nonprofit environmental and conservation groups. The Swim Guide is available as an app for iPhone or Android and provides real-time updates on beach quality and safety for more than 7,000 beaches, lakes, rivers and swimming holes in the US, Canada, Mexico, the Bahamas, Ireland, France, New Zealand and Australia.

Also: Avoid swimming if a discharge pipe can be seen on the beach…there are dead fish or other animals in or near the water…the water is discolored, smelly, foamy or scummy.

Lakes

Various kinds of E. coli, including the Shiga Toxin-producing Escherichia coli (STEC), can contaminate lakes. E. coli are bacteria that live normally in the intestines of humans and animals, and most are harmless. However some, such as STEC, can cause illness. Since the sources of contamination can be human and/or animal—not just pets but also farm and wild animals, such as cattle or racoons—lakes that are used for recreational purposes are regularly tested. STEC infection can cause diarrhea, vomiting and cramps. Racoons pose a particular risk—their feces can contain eggs from Baylisascaris procyonis, a worm that can infect humans, especially children, and potentially cause severe disease that affects the nervous system or eyes.

Best defense: Do not swallow the water when swimming or playing in lakes. Do your research beforehand to see if bacteria levels are safe for swimming. Protect backyard pools from racoons (and other animals), such as with an animal-proof cover. For small wading pools, drain the water when not in use.

If you notice symptoms—such as feeling ill, skin rash and/or eye irritation—after swimming or playing in water, see a medical doctor immediately and let him/her know that you were recently exposed to potentially contaminated water.

Special Concerns

Lakes, ponds, streams and other waterways, both salt and fresh, can become contaminated with blue-green algae blooms during long spells of warm weather. The algae can produce a toxin that can make humans and pets very sick—even if you or your pet don’t get into the water, because breathing in mist or droplets of the water can also make you sick.

Not all algae are toxic. But there’s a good chance algae is toxic if the water is green or brown…cloudy or thick (“pea soup” appearance)…looks like streaks of blue, green or white paint is spilled on the surface…looks scummy or foamy…smells bad.

Best defense: Often, toxic algae blooms are identified by public health officials and warnings are posted to stay out of the water. If you do see such a warning posted, keep far enough away to avoid getting splashed or breathing in vapor from the water.

“Flesh-eating” bacteria. Frightening news items notwithstanding, picking up a so-called flesh- or brain-eating microbe while swimming is—fortunately—rare.

One type of “flesh-eating” bacteria is Vibrio. Vibrio bacteria live in certain coastal waters year-round but are in higher concentrations when temperatures are warmer (typically, May to October). There are several strains of Vibrio, including one that causes cholera, but the most common strains in the US cause the human illness vibriosis. You can get vibriosis from eating raw or undercooked seafood, especially oysters, or by getting seawater into a wound. People with compromised immunity—especially people with liver disease—are at higher risk from exposure.

Best defense: Avoid salt water if you have an open wound. For instance, wear protective gloves when shucking oysters. Also, don’t eat raw or undercooked shellfish.

Naegleria fowleri, aka the brain-eating amoeba, infects people when contaminated water enters the body through the nose when putting your head under water.

Best defense: Even though this infection is rare, since the amoeba’s habitat is warm waterways, the only sure way to prevent exposure is to avoid swimming in freshwater lakes, ponds and rivers during periods when water temperature is high…and to give hot springs and other untreated thermal waters a miss. If you do take part in water-related activities in bodies of warm water, avoid putting your head under water…and wear a nose clip.

Neti pots: Because amoebae can also be found in tap water, only distilled or sterile water should be used in neti pots. And neti pots should be thoroughly cleaned before use, either with soap and hot water or by running them through the dishwasher.