Warts are small, rough growths that often occur on the hands and feet, but may occur on other parts of the body. Warts are caused by a viral infection in the top layer of skin. Because warts occur as a result of a virus, warts can be passed from person to person, sometimes indirectly. Warts can disappear without treatment; however, warts that are painful or rapidly multiplying should be treated. Numerous salicylic acid treatments are available over the counter. Common physician-performed treatments include application of cantharidin or liquid nitrogen, and excisions.