Common warts (Verruca vulgaris) on a patient's hands. Warts are harmless, contagious growths of the outermost layer of the skin (epidermis). They are caused by the human papilloma virus (HPV), of which there are over 30 types. Some warts spontaneously disappear within a year. Others can be removed with chemicals or lasers or by freezing (cryotherapy) or burning (cautery).