When skin warts appear, it can seem as if the harmless growths came out of nowhere.

But common warts are actually an infection in the top layer of skin, caused by viruses in the human papillomavirus, or HPV, family. When the virus invades this outer layer of skin, usually through a tiny scratch, it causes rapid growth of cells on the outer layer of skin – creating the wart.

“HPV is ubiquitous,” says dermatologist Conway Huang, MD, an associate professor of dermatologic surgery and cutaneous laser surgery at the University of Alabama at Birmingham. “We all come in contact with it,” throughout our lives, such as when shaking hands, turning doorknobs, or typing on keyboards.

Scientists have identified more than 100 unique types of the virus. And most people will have at least one common wart at sometime in their lives, usually on their hands.

Certain forms of the virus are more likely to cause skin warts on the hands. Other forms of HPV are more likely to cause genital warts, although some strains of the virus can cause both.