Papillomaviruses are a diverse group of DNA-based viruses that infect the skin and mucous membranes of humans and a variety of animals (replicating exclusively in keratinocytes). More than 100 different human papillomavirus (HPV) types have been characterized. Some HPV types cause benign skin warts, or papillomas, for which the virus family is named. HPVs associated with the development of such "common warts" are transmitted environmentally or by casual skin-to-skin contact. A separate group of about 30 HPVs are typically transmitted through sexual contact. Genital HPV infection is very common, with estimates suggesting that up to 75% of women will become infected with one or more of the sexually transmitted HPV types at some point during adulthood. HPV infection is a necessary factor in the development of nearly all cases of cervical cancer. Cervical Pap smear testing is used to detect HPV-induced cellular abnormalities.