Warts are benign proliferations of skin and mucosa caused by the human papillomavirus (HPV). Currently, more than 100 types of HPV have been identified. Certain HPV types tend to infect skin at particular anatomic sites; however, warts of any HPV type may occur at any site. The primary clinical manifestations of HPV infection include common warts, genital warts, flat warts, and deep palmoplantar warts (myrmecia). Less common manifestations of HPV infection include focal epithelial hyperplasia (Heck disease),  epidermodysplasia verruciformis, and plantar cysts. Warts are transmitted by direct or indirect contact, and predisposing factors include disruption to the normal epithelial barrier.
Treatment is difficult, with frequent failures and recurrences. Many warts, however, resolve spontaneously within a few years even without treatment.
A small number of high-risk HPV subtypes are associated with the development of malignancies, including types 6, 11, 16, 18, 31, and 35. Malignant transformation most commonly is seen in patients with genital warts and in immunocompromised patients. HPV types 5, 8, 20, and 47 have oncogenic potential in patients with epidermodysplasia verruciformis.
Warts can affect any area on the skin and mucous membranes. The HPV virus infects the epithelium, and systemic dissemination of the virus does not occur. Viral replication occurs in differentiated epithelial cells in the upper level of the epidermis; however, viral particles can be found in the basal layer.
Warts are widespread in the worldwide population. Although the frequency is unknown, warts are estimated to affect approximately 7-12% of the population. In school-aged children, the prevalence is 10-20%. An increased frequency also is seen among immunosuppressed patients and meat handlers.
Although warts may affect any race, common warts appear approximately twice as frequently in whites as in blacks or Asians.  Focal epithelial hyperplasia (Heck disease) is more prevalent among American Indians and Inuit. 
Male-to-female ratio approaches 1:1.
Warts can occur at any age. They are unusual in infancy and early childhood, increase in incidence among school-aged children, and peak at 12-16 years. 
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