Common wart

A common wart, known scientifically as verruca vulgaris, is a benign skin lesion. Benign means it isn't cancer. It's caused by the human papillomavirus, or HPV for short. It looks like a papule, a small raised bump with an uneven edge, and it most often shows up on the extremities, meaning the arms and legs.

Here's what happens beneath the surface. HPV spreads through contact, and it gets in more easily when the skin's barrier, the outer layer that protects us, is damaged in some way. What it infects is the epidermis, the top layer of the skin, and that layer starts to overgrow. But it doesn't grow evenly. It rises into finger-shaped projections, which is called digitated hyperplasia. Picture the bristles of a brush up close: lots of little tips standing up and packed tightly together. Running through those fingers are capillaries, the body's tiniest blood vessels, and inside a wart they're twisted.

When the surface of a wart is pared down, tiny black dots often appear. These are thrombosed capillaries, which means tiny vessels plugged by a clot. And here's the good news: about two-thirds of warts go away on their own, and they leave almost no scar.