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Wart treatment. Wart on a finger being killed by liquid nitrogen on a cotton bud. The use of cold to kill tissue is known as cryosurgery. The liquid nitrogen freezes the wart solid. As the wart thaws, a blister forms, lifting it off the skin. Warts are growths on the skin caused by the human papillomavirus (HPV). Photographed during a wart clinic at a general practice doctor's surgery. (KEYSTONE/SCIENCE PHOTO LIBRARY/ANTONIA REEVE)