About common warts
The causative agent of the common wart is human papillomavirus (HPV). The virus that enters the skin causes an overproduction of keratinised tissue. This results in a dome of callus which protects & hides the virus and allows the wart to keep growing.
About plantar warts (verrucas)
Plantar warts (or verrucas) are also caused by the human papillomavirus. They are very common, being experienced by most people at some time during their lives. While common warts remain as a dome on top of the skin, verrucas are pushed back into the skin by the pressure of walking on them. This inward growth can make plantar warts more painful and difficult to treat.
Warts are easily transmitted by direct contact with a human papillomavirus. The virus is able to enter the skin through minor breaks/cuts in the skins surface. Warts are less common in adults because most adults will have developed immunity to strains of the HPV virus, while children are still susceptible. Infection can be passed on to others by sharing towels, razors, or other personal items. After contact with HPV, it may take many months of slow growth beneath the skin before you notice a wart.