Common warts, like other types of warts, are caused by the human papillomavirus (HPV). There are more than 100 different types of HPV, and a large number of them can cause skin warts.
A wart begins forming when HPV infects the top layer of skin. A break in the skin (such as a scrape or cut) makes it easier for the virus to enter the body. The infection causes skin cells to multiply rapidly. Despite this rapid growth of skin cells, the wart forms so gradually that it could take anywhere from several weeks to nine months before you can actually see the wart on the surface of your skin.
It's possible that immunity to warts increases with age. Young children are more susceptible to common warts than adults are, and girls are more likely to develop warts than boys. Experts say that about 10 percent of children will get common warts, and those who do get them are typically between the ages of 12 and 16 [source: Bacelieri]. Children who bite their fingernails or chew hangnails are more susceptible to acquiring warts. The skin abrasions caused by biting and chewing the fingers and nails create perfect entry points for the HPV virus.
You can take some precautions to avoid getting warts, but it's impossible to ensure you won't get them. Read on to learn about how common warts spread.