Warts are non-cancerous skin growths caused by a viral infection in the top layer of the skin. Viruses that cause warts are called Human Papilloma Virus (HPV). There are several different kinds of warts including Common warts which usually grow on the fingers or around the nails where the skin has been broken, for example where fingernails are bitten or hangnails picked. These are often called "seed" warts because the blood vessels to the wart produce black dots that look like seeds. Plantar warts are usually on the feet and grow in clusters. Flat warts are smaller and smoother and tend to grow in large numbers and can occur anywhere.