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. 
Approximately 65% of warts disappear spontaneously within 2 years. When warts resolve on their own, no scarring is seen. However, scarring can occur as a result of different treatment methods. Growth of periungual or subungual warts may result in permanent nail dystrophy.
Treatment failures and wart recurrences are common, more so among immunocompromised patients. Normal appearing perilesional skin may harbor HPV, which helps explain recurrences.
Common warts are usually asymptomatic, but they may cause cosmetic disfigurement or tenderness. Plantar warts can be painful, and extensive involvement on the sole of the foot may impair ambulation. Malignant change in nongenital warts is rare but has been reported and is termed verrucous carcinoma. [4, 5, 6] Verrucous carcinoma is considered to be a slow-growing, locally invasive, well-differentiated squamous cell carcinoma that may be easily mistaken for a common wart. It can occur anywhere on the skin but is most common on the plantar surfaces. Although this type of cancer rarely metastasizes, it can be locally destructive.
Alert patients to the risk factors for transmission of warts. These include trauma or maceration of the skin, frequent wet work involving hands, hyperhidrosis of feet, swimming pools, and nail biting. Butchers and slaughterhouse workers also are at increased risk for developing warts.
Alert patients that some warts may require multiple treatments and may be resistant to several treatment modalities. In addition, some warts may regress spontaneously without treatment.
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