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If your summer has ever included cooling off with a nice swim in a lake, you might be acquainted with something called swimmer’s itch. These itchy red bumps appear on the exposed parts of your skin after you take a dip in a lake or pond, and they can last for a week or more.

But swimmer’s itch is more than just an annoying rash it’s actually caused by parasites that burrow into your skin and then die there. Yeah. Ew.

The culprits are tiny worms called schistosomes. Each schistosome species specializes in a specific bird or mammal host. Some schistosome species target humans, causing a debilitating disease called schistosomiasis.

But the ones that cause swimmer’s itch aren’t after you they’re part of a different group of species whose hosts include ducks, geese, muskrats, and raccoons. That itchy rash is what happens when one of these schistosome larvae makes a mistake. Adult parasites live in their host’s blood, and when they lay eggs, the host eventually poops them out.

With a little luck, the eggs end up in water, where they hatch into larvae that swim around in search of the aquatic snails that they need to infect to complete the next stage of their life cycle. The baby schistosomes continue to multiply and develop inside the snail, and eventually the infected snail releases a second type of larvae called cercariae into the water. This is where swimmer’s itch gets its technical name, cercarial dermatitis.

These little guys, each less than a millimeter long, head out to look for a member of their original host species to start the cycle all over again. You’d think it would be easy to tell the difference between a human and a muskrat or goose or whatever, but sometimes the cercariae mess up and burrow into a human swimmer’s skin. It’s a fatal mistake.

They can’t develop there, and so they die. And because dead baby parasites are definitely not something that’s supposed to be in your body, they trigger an allergic reaction as they break down. That’s what all the itching and redness is from.

Because it’s an allergic reaction, some people are more sensitive than others it just depends on how strongly your immune system responds. And if you’re exposed to swimmer’s itch over and over again, you can actually become more sensitive to it over time. You’re most likely to get swimmer’s itch in freshwater, but it’s possible to get it in salt water too, so even at the ocean you’re not completely safe.

To keep your risk small, towel off or shower as soon as you get out of the water, and don’t do anything to attract birds that may host schistosomes to the area. That’s right -- no more feeding the ducks. It’s a small price to pay to avoid parasites dying a slow, itchy death under your skin.

If you don’t want to take your chances at the beach, maybe a mathematical game of chance is more your style. This course on Brilliant.org will sharpen your math skills while helping you master probability. Thinking about parasites dying under human skin really put me in the headspace for the “Survive this Chapter” part.

You have to avoid eating a poisoned apple and all kinds of dangerous situations to progress onto the next challenge. It’s fun, and I learned a lot, and best of all, you’re actually completely safe the whole time. Check it out at Brilliant.org/SciShow and right now, the first 200 people to sign up at that link will get 20% off of an annual premium subscription to Brilliant. [♩OUTRO].