Plane warts on a patient's forehead. Warts are contagious, yet harmless, skin growths. They are caused by the human papilloma virus (HPV). There are at least thirty different types of HPV. Plane warts are flat and skin-coloured and are usually found in large numbers on the face. Warts affect only the outermost layer of skin. Around half of them spontaneously disappear within a year. Warts can be removed using chemicals or lasers, or by freezing (cryotherapy), scraping (curettage) or burning them (cautery).