Warts can appear anywhere on the body. When they are on the bottom of your foot, they are called plantar warts because they occur on the sole of your foot.
Because the bottom of the foot has a lot of pressure, the warts grow inward into the tissue of your foot. Sometimes they are covered by a callus that you can see on the surface of the skin.
Plantar warts are caused by the human papillomavirus which enters the body through a break in the skin. This virus is very contagious and can be picked up in public areas such as swimming pools, showers and locker rooms. Teenagers and children are prone to getting this virus as are those with weakened immune systems.