If you’ve just finished a nice day in the ocean in warm, tropical waters only to find itchy red bumps anywhere your bathing suit may have been rubbing, you might have been told to blame sea lice. In reality, “seabather's eruption” is actually caused by the stings of the tiny larvae of thimble jellyfish and sea anemones.

These larvae get stuck in your bathing suit and in your hair, and tiny stinging organelles called nematocysts in their cells get triggered and sting you when they are exposed to friction or fresh water.

Because the larvae are about the size of a speck of finely ground pepper, they’re almost invisible in the water and easily get stuck in between your bathing suit and your skin.

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This is why it can actually be better to swim with less clothing and coverage; it can minimize the opportunities for the larvae to come with you when you exit the water.

Unfortunately, you can’t exactly opt to wear your - ahem - birthday suit instead of a bathing suit at many beaches, but there are a few other things you can do to minimize your risk.

Identifying and minimizing the risk:

Seabather’s eruption from thimble jellyfish is especially common in tropical and subtropical waters, including the Caribbean and along the U.S.’s Southeast and Gulf Coast regions and in the Bahamas. Cases can reach farther north, though, like in the summer of 2020 when Hurricane Isaias pushed the jellyfish larvae all the way to New Jersey and caused an outbreak.

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Sea anemone larvae are more common further north, along the East Coast from the mid-Atlantic to New York.

It is also more common in the spring and summer months, especially May and June, around breeding peaks when there are more larvae in the water.

They can be unpredictable, though, because they drift with ocean currents and won’t necessarily be a problem at the same beaches from year to year.

Look for a purple flag flying at the beach before you go in the water, because it means there’s dangerous marine life present, so you should proceed with caution.

Symptoms and how to treat them:

If you’re swimming somewhere seabather’s eruption is common, the first thing you should do when you get out of the water is remove wet clothing and anything else that could provide a nook or cranny for the larvae to hitch a ride in.

You should actually do this before rinsing off in freshwater, because exposure to freshwater can trigger the sting. Any larvae left over that didn’t come off when you took your bathing suit off can be washed away before they get the chance to sting you.

If you wait to get clean, you increase the chance of something rubbing a spot where the larvae are hiding out, and that kind of friction can also make them sting you, which leads to a rash.

The rash typically develops within minutes to a day of exposure, and it consists of raised, red bumps on the skin, usually in areas that were covered by your bathing suit.

Antihistamines, hydrocortisone cream and cold packs can help soothe the rash once it appears, and they usually clear up within about two weeks as long as you take care of it. As hard as it is, don’t scratch, because you can cause open wounds and even infections.

In rare cases, more symptoms are possible, including fever, nausea, vomiting, cramps and a general feeling of being unwell.

If symptoms don’t get better or if you notice signs of an infection, make sure you see a doctor.

Sara Tonks is a content meteorologist with weather.com and has a bachelor’s and a master’s degree from Georgia Tech in Earth and Atmospheric Sciences, along with a master’s degree from Unity Environmental University in Marine Science.