Warts are caused by the human papilloma virus, which enters the skin surface, causing growth and thickening of the top (epidermal) layer of the skin. This produces the rough, hard texture of a wart. Warts appear most often on the hands and feet, though they can spread to other parts of the body, for example around the nails, lips and genitals. Most people will have warts at some point in their life, though they tend to affect children and young adults. Warts are contagious, and close skin-to-skin contact can pass on the infection. The risk of infection is increased if the skin is damaged or wet and in contact with roughened surfaces.