It’s swimming season! As the temperature rises, water (both for drinking and submerging in) becomes more seductive. And when there is more swimming, there are more swimming hazards. When we think about water safety, our thoughts go straight to drowning (or sharks, if we are ocean-bound). But summer swimming exposes us to a less deadly but more common and still serious threat. So, let’s wade (sorry!) into the topic of waterborne illnesses, the ones that make your stomach churn—sometimes literally.

More than seven million of us (1 in 44) experience a waterborne illness every year. The illnesses range from miserable but minor gastrointestinal problems to rare, flesh-eating bacterial infections that can cost a limb—or a life.

Swimming pools (regular and kiddie), lakes, hot tubs, and even splash pads all have one thing in common: people. And people, as it turns out, can add germs to any standing water in a variety of ways. And while there are germs everywhere, germs that survive in standing water can easily ruin a fun summer weekend.

The most common swimming-related illnesses are caused by microscopic party crashers: bacteria, viruses, and parasites. They can hang out in natural water and treated water—even when it looks sparkling clean.

Here are the big offenders:

• Cryptosporidium (often called “crypto”) – This parasite is the reigning champ of pool-related illnesses. It causes diarrhea that can last up to three weeks. It’s chlorine-resistant, meaning it can survive even in well-maintained pools.
• Giardia – Another parasite, often found in lakes and rivers. Like crypto, it causes prolonged diarrhea, and just a mouthful of contaminated water can do the trick.
• Vibrio vulnificus – Most often found in lakes, rivers, and oceans, this is one of a category of bacteria that can sneak into cuts, wounds, or even irritations and cause serious infections—including what people often call “flesh-eating bacteria.” It’s rare—but when it happens, it can spread fast, especially in people with weakened immune systems or conditions like diabetes.
• E. coli – Yes, the same bacteria we hear about in undercooked hamburgers. It can also thrive in warm, poorly treated water. And it doesn’t take much—just a few cells can make you sick.
• Norovirus – That cruise ship virus? It’s also a pool party crasher. It spreads fast and makes people very sick with vomiting and diarrhea.
• Pseudomonas – Found in hot tubs and poorly chlorinated pools, this can cause “hot tub rash” or “swimmer’s ear.” It’s not life-threatening, but it’s annoying, itchy, and decidedly unsexy.
  
  

Why does this happen in pools treated with chlorine?

Every time someone gets in the water, they (we) bring along a few… extras: sunscreen, sweat, body oils, soap residue, makeup, and microscopic traces of fecal matter (yes, really).

Chlorine has to work overtime to neutralize all that gunk. But the more organic material it has to handle, the less of it is available to kill actual harmful germs like crypto or E. coli. It’s like trying to mop up a flood with one towel—it might be the right tool, but it can only handle so much.

And remember, some parasites are highly chlorine-resistant to begin with. If the sanitizing agent is already maxed out just breaking down sunscreen and sweat, those hardier microbes get a free ride.

That’s why it’s not just about how much chlorine is in the water, it’s about what that chlorine is being asked to do. A pool might technically be within the recommended chemical levels and still be risky if it’s been overrun with swimmers or not given enough time to recover between sessions.

It’s not just public pools, either—backyard pools, splash pads, water parks, lakes, rivers, and yes, even some “natural hot springs” can harbor these invisible invaders.

To sum it up: the water may look clear, but the microbes don’t always RSVP in visible ways.

How to Protect Yourself

You don’t have to stay dry all summer to stay safe. But a few habits go a long way:

• Don’t swallow the water. REALLY! TRY NOT TO INGEST WATER THAT YOU SWIM IN. Swallowing water, even a tiny amount, is the single most common cause of all gastrointestinal waterborne illnesses. If you inadvertently get swimming water in your mouth, spit it out and rinse your mouth.
• Shower before you swim and after. You know those “shower before entering” signs at resort and public pools that almost everyone ignores? There’s actually a good reason to follow that rule. A quick rinse gets rid of sweat, lotion, and any microscopic stowaways, which can overwhelm a pool’s sanitizing system. Rinsing for a minute before swimming will help keep your fellow swimmers safe. Rinsing for a minute afterward will help keep you safe.
• Don’t pee in the pool. This is pretty much a given, but it’s a rule some people break figuring that chlorine will take care of that minor infraction. But pee is one of those things that can overwhelm the sanitizing agent. So, peeing in the pool is worse than just rude.
• Don’t swim if you’ve had diarrhea in the last 2 weeks. It doesn’t matter how many showers you’ve taken since then. You’re still carrying the germs that caused the condition, which can spread easily in a pool. That goes double for kids in swim diapers—they’re not watertight.
• Do not swim with even slightly broken or irritated skin. Any break in your skin—scrapes, shaving nicks, new piercings, healing tattoos—can be an open door to bacteria. When in doubt, cover it up with a waterproof bandage or just skip the swim.
• Check pool inspection scores when you can (most public pools post them online).
• At lakes or rivers, avoid swimming near storm drains or after heavy rains—runoff can carry waste and bacteria into the water.
• Keep hot tubs hot and chlorinated. A warm tub is basically a microbe Jacuzzi if it’s not properly maintained.
  
  

Wait—are hot tubs worse than pools?

Surprisingly, yes. Hot tubs are like a spa day for bacteria. Here’s why:

• Warm water weakens chlorine. That soothing 100°F water you’re soaking in? It breaks down disinfectants faster, making it harder to kill off microbes.
• It’s a people soup. Small space, lots of bodies, and not a lot of circulation. Sweat, skin cells, and, ahem, other body stuff can quickly overwhelm the sanitizer.
• Bacteria love it here. Pseudomonas is especially fond of hot tubs and can lead to red, bumpy rashes—especially in areas covered by your swimsuit.
• Long soaks = more exposure. People tend to sit in hot tubs longer, which means more time for germs to find their way into ears, eyes, or other damp crevices.
• Maintenance matters. Hot tubs require even more attention than pools, but they’re often neglected. If the water smells like a weird chemical cocktail or looks cloudy, take a pass.
  
  

Final Thought

Most waterborne illnesses aren’t life-threatening—but they can seriously ruin your vacation, your appetite, and your faith in humanity. The good news is a little awareness can go a long way. Don’t be reckless with other people’s health. Swim smart, rinse well, and definitely—keep your mouth closed underwater this summer.