Warts that occur on the bottom of your feet are called plantar warts. Often they occur on the areas of the foot that experience the most pressure, such as the ball of your foot or the heel. Because of the increased pressure to those areas from walking and physical activity, plantar warts in these areas often penetrate deep into the tissue and can be very painful. Warts are caused by a virus known as human papilloma virus, or HPV. They can be spread in moist environments such as public showers, locker rooms and swimming areas, but not everyone who comes in contact with a plantar wart will develop one. Genetic predisposition and stress can increase your risk of developing warts on your feet.