When baseball star Bryce Harper, an outfielder for the Washington Nationals, tweeted a picture of his bloody big toe and a piece of the ingrown toenail that was removed from it, some cringed at the sight.

But this commonplace foot problem affects about 18 percent of U.S. adults age 21 and older—that’s about 40 million folks. Ingrown toenails happen when the corner or side of the nail grows into the soft flesh of the toe. More often than not, it’s the big toe that suffers.

To treat an ingrown toenail, the Institute for Preventive Foot Health suggests you soak your feet in warm water for 15 to 20 minutes several times a day to reduce swelling and relieve tenderness. Next, clean the area with alcohol.

Next, use a clean file to lift the nail up and slightly away from the skin. Place cotton under the edge of the nail to separate it from the skin. (Change the cotton each day.) Finish by swabbing again with alcohol. This will train the nail to grow back above the edge of the skin.

To avoid the problem, cut nails straight across, and don’t wear tight-fitting shoes or footwear that squeezes your toes together.