What causes warts on hands? Here’s what you need to know.

Hand warts are an easily contractible skin condition that can be passed at even the most mundane of moments, from shaking someone’s hand to sharing towels at the pool.
Warts on hands are caused by an infection of the human papillomavirus (HPV). There are more than 100 different strains of HPV, according to the U.S. Food & Drug Administration. Different strains of HPV will impact different areas of the body, and not all types of the virus cause warts.
When your skin comes in contact with a wart-causing strain of HPV, the virus will prompt your body to produce extra cells. The result is a thick, hard growth on the outer layer of your skin, according to WebMD.
Common warts, known as verruca vulgaris, often appear on the hands, says Dr. Ronda Farah, MD, FAAD, a dermatologist with M Health Fairview and the University of Minnesota Medical School. These warts usually look like rough, skin-colored bumps, she says. Common warts may have black dots that resemble seeds. They’re mostly caused by HPV strains 2 and 4, although they’ve been linked to a number of other types of HPV, including 1, 3, 7, 27, 29 and 57, per Cleveland Clinic. 
Flat warts can also appear on the hands. These warts manifest as “skin-colored little bumps that disrupt the lines of the skin,” and they’re usually located on the “tops of the hands,” says Farah. In contrast to most warts, flat warts have a smooth appearance. They have the potential to grow in clusters of 20 to 200. They’re caused by HPV strains 3, 10, 28, and 49, according to Healthline.
Periungual warts, which grow in the skin surrounding fingernails and toenails, are a reaction to  HPV strains 1, 2, 4, 5, 7, 27 and 57, per WebMD. Butcher’s warts commonly occur in people who regularly handle raw meat or fish, and they’re caused by HPV strain 7, according to Cleveland Clinic.
“All warts are contagious,” Farah says. Warts can spread if you touch a surface that someone with a wart has also touched, she says. Warts can also be easily passed from skin-to-skin contact, and can also transfer from one body part to another, according to Mount Sinai. 
While contagious, warts are usually harmless, painless skin growths. “Warts are typically not cancerous, they’re benign,” explains Farah. Rarely, people can develop verrucous carcinoma, a type of skin cancer. In the event that your wart is rapidly growing and is unresponsive to treatment, or if you are experiencing pain, a board-certified dermatologist can perform a biopsy, Farah says. 
Common warts that appear on the hands are not considered to be an STD, Farah says. It’s important to distinguish that the strains that cause genital warts are not the same as the strains that cause common warts, Farah says. 
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There are over 40 kinds of HPV that can impact the genital area, per Mayo Clinic. The strains of HPV that cause genital warts are almost always passed through sexual contact. HPV is one of the most common sexually transmitted infections in the United States, and it’s estimated that over 14 million Americans are newly infected with it each year, per Cleveland Clinic.