an icosahedral DNA virus, 55 nm in diameter, of the genus Papillomavirus, family Papovaviridae; certain types cause cutaneous and genital warts; other types are associated with severe cervical intraepithelial neoplasia and anogenital and laryngeal carcinomas. Over 70 types have been characterized on the basis of DNA relatedness. SYN: infectious papilloma virus.HPV infection is the most common viral sexually transmitted disease. A single unprotected contact with an infected person carries a 60% risk of infection. The interval between exposure and clinical evidence of disease ranges from 3 weeks to 8 months. The annual incidence of genital HPV infection in the U.S. is estimated to be 3.5 million cases, with a prevalence of current infection of 20 million. More than one half of all sexually active women have been infected with one or more genital HPV types. About 15% have DNA evidence of current infection and 1% have genital warts. Most HPV infections are subclinical and transitory, the median duration of infection being about 8 months, with persistence rates of 30% at 1 year and 9% at 2 years. However, some types of HPV can induce genetic mutations in cervical epithelium that can culminate, after a latent period of 10-20 years, in the development of cancer. Carcinoma of the cervix is the most common malignancy in women under age 50. In the U.S., the incidence of cervical cancer is 8.3/100,000, with approximately 14,000 cases and 1,000 deaths annually. As many as 98% of all cancers of the cervix (most of which are squamous cell carcinomas) are believed to be induced by infection with HPV types 16, 18, 31, 33, and a few others. HPV typing in women with atypical squamous cells of undertermined significance (ASC-US) on cervical Papanicolaou smear helps to identify those in whom more intensive surveillance for premalignant change is warranted. Women with external genital warts (condylomata acuminata, which are usually due to HPV type 6 or 11) are not at increased risk of cervical cancer and do not need special surveillance if routine Papanicolaou smears are negative. Some 40% of HIV-positive women develop severe cervical dysplasia caused by HPV, which in many cases proceeds to fatal cancer with an aggressiveness not commonly seen among non-HIV-positive women. Diagnosis of HPV infection is based on visual inspection (including colposcopy with application of acetic acid to the cervix), Papanicolaou smear, and biopsy, with detection of viral DNA in tissue. Treatment options depend on the site and extent of involvement and include surgical excision, cryotherapy with liquid nitrogen, topical application of bichloracetic acid or trichloracetic acid, laser ablation, loop electrosurgical excision, and intralesional injection of interferon. External genital warts usually respond to topical treatment with podofilox gel or imiquimod (a cytokine-inducing agent that can be applied by the patient). Subclinical HPV infection, detectable only by Papanicolaou smear or other laboratory methods, may prove impossible to eradicate. The virus cannot be cultured, and there is no test to confirm cure. In limited trials, a vaccine has demonstrated efficacy in preventing both HPV-16 infection and HPV-related cervical intraepithelial neoplasia in women not previously exposed to the virus.