Warts are caused by the human papillomavirus (HPV). HPV is spread through direct, skin-to-skin contact, as well as through indirect contact. The virus causes skin cells in the epidermis, the uppermost of the three layers of skin, to divide at an exaggerated pace, thus creating the growth. There are more than 100 strains of HPV and more than half of these can cause warts.
Common Wart
HPV infects skin at sites where the skin has been broken or damaged. For that reason, verrucae vulgaris, known as common warts, are often found on the fingers, the backs of hands and on knees and elbows. Although anyone who comes into contact with HPV can potentially develop common warts, they more often develop on children and teenagers than on adults.
Not everyone who comes into contact with HPV will develop warts. In a column in "Medical Edge Newspaper," Mark Dahl, of Mayo Clinic in Scottsdale, Arizona, explains that some people are immune to HPV, while others develop immunity.
Common warts appear as a rough, raised bumps on the skin. They can be flesh colored, although they are often white or pink. Common warts are sometimes described as "cauliflower-like" due to their knobbed appearance. Because they spread easily, common warts are often found grouped together in clusters.
Common warts often have tiny black or dark spots interspersed throughout their surface, which are the blood vessels that feed and nourish the growths.
Periungual and Subungual Warts
Warts that develop in the skin around fingernails and toenails are called "periungual warts." Warts that grow beneath the nails are called "subungual warts." People who bite their nails or who pick at their cuticles often break or damage the skin around their nails, which allows HPV to enter and infect the site.
Subungual warts are especially difficult to treat, as most wart treatments require direct application of a medication or therapy to the wart itself. If left untreated, subungual warts can permanently deform the nail bed. Because of this, medical practitioners often remove the nail to access and treat the wart.
Flat Warts
Flat warts are smoother and smaller than other kinds of warts. Unlike common warts, flat warts are only slightly raised above the surrounding skin. Clusters of 50 to 100 flat warts are not uncommon.
This type of wart is often, though not always, found in areas where the skin is shaved, such as in the beard area and on the legs. The scraping action of the razor opens new sites where the virus can infect skin.
HPV can infect skin that is dry and chapped, which is why flat warts commonly form on the backs of hands.
