This chapter addresses the natural history of anogenital human papillomavirus (HPV) infection. Cervical infections are the best understood HPV infection. Cervical HPV persistence is the known necessary event for the development of cervical cancer. New infections appearing at any age are benign unless they persist. Several long-term natural history studies have now shed light on the very low risk of cervical intraepithelial neoplasia (CIN) 3+ in women past the peak of HPV acquisition (e.g., 30 or older) who are HPV-negative or clear their HPV. Although data on transmission of HPV are finally emerging, rates of transmission between heterosexual couples vary widely among studies. Factors that affect the calculations of these rates include a) intervals between testing points, b) rates of concordance or discordance at baseline, and c) difficulty in defining established infections versus contamination. Both cervix to anus and anus to cervix autoinoculation in the same woman appears to be quite common. Whether either site serves as a long-term reservoir is unknown. Studies show that anal infections in women and in men who have sex with men are quite common with cumulative rates up to 70-90%. Similarly, clearance of anal HPV is also common, with few individuals showing persistence unless they are human immunodeficiency virus (HIV)-infected. HIV strongly influences the development of anal intraepithelial neoplasia (AIN). The few studies on the natural history of AIN in HIV-infected men suggest that high-grade AIN is a precursor to invasive anal cancer. Although no natural history studies of AIN are available in women, women with other HPV-associated lesions, including CIN3+ and vulvar cancer, have higher rates of anal cancer. Data on the natural history of HPV of the male genitalia are also emerging, although penile intraepithelial neoplasia is poorly understood. Cumulative rates of HPV are extremely high in men and risks are associated with sexual behavior. Unlike women, prevalence rates are steady across all ages, suggesting that men do not develop protection against reinfection. This article forms part of a special supplement entitled "Comprehensive Control of HPV Infections and Related Diseases" Vaccine Volume 30, Supplement 5, 2012.
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