Warts are caused by viruses, specifically HPV (human papillomavirus). The virus causes keratin, which is a hard protein on the top layer of the skin, to grow too fast. Warts are not the same as moles. Moles are typically dark while warts are often the same color as the person's skin. The appearance of a wart can vary depending where it is on the body and how thick the skin is. A wart that is located on the sole of the foot is known as a verruca. Contents of this article: Causes.