Viral wart. Light micrograph (LM) of a section through a wart caused by a virus in a person's skin, showing the exophytic papillary form. The epidermis (yellow) is irregularly thickened and covered by a layer of hyperkeratosis. The dermis (light pink, bottom) shows chronic infiltration of inflammatory cells. Viral warts are benign proliferations of the skin and mucosa caused by infection with human papillomavirus (HPV); more than 150 subtypes of the DNA virus are known. Viral warts are particularly common in childhood (affecting about 15% of school children) but may arise at any age. They are spread by direct contact or autoinoculation with a latency of weeks to years. They are frequently painful and they cause significant embarrassment. Magnification: x 100 when printed at 10 centimetres high.