Warts result from infection by certain viruses called papillomaviruses.
The viruses live in cells on the surface layer of the skin and do not infect the underlying tissue. The thickened surface layer forms folds into which little blood vessels grow spontaneously, or if a wart is scratched open, the viruses may spread by contact to another part of the body or to another person. Common warts vary in color from normal flesh tint to dark brown-black. They usually occur on the hands, elbows, knees, and less frequently on the face or eyelids. They may also appear around the edges of the nails.