According to WebMD, common warts are caused by viruses in the human papillomavirus, or HPV, family. A wart is created when the virus invades the outer layer of skin, usually through a tiny scratch or cut in the skin.
A wart is a small growth that appears at the point of contact with the virus. The virus causes keratin, a hard protein, to grow on top of the skin, giving a wart the appearance of a solid blister or cauliflower, according to Medical News Today. Warts appear most commonly on the hands and feet, though they may also appear on other parts of the body that come in contact with the virus. Most people have at least one common wart in their lives, usually on their hands, according to WebMD.