Common wart on a patient's finger. A wart is a contagious, yet harmless skin growth. It is caused by the human papilloma virus (HPV). There are over thirty different types of HPV. A common wart is a firm horny growth that can reach around one centimetre across. It is usually found on the back of a hand. Warts affect only the outermost layer of skin. Around half of them spontaneously disappear within a year. Warts can be removed with chemicals or lasers, or by freezing (cryotherapy), scraping (curettage) or burning them (cautery).