Warts can occur anywhere but they are seen most often on the fingers, the backs of the hands, knees, and elbows. These are sites that most often get breaks in the skin like cuts or abrasions that expose the lower layers of the epidermis to the human papillomavirus (HPV). Because the virus only stays in the epidermis
and doesn't penetrate into the dermis where the blood vessels are located, often the immune system doesn't even know the wart is there. This is one of the reasons it can be difficult to get rid of a wart.