The human papilloma virus types 1, 2, 4, or 63 are classified as clinical (visible symptoms). The virus attacks the skin through direct contact, entering through possibly tiny cuts and abrasions in the stratum corneum (outermost layer of skin). After infection, warts may not become visible for several weeks or months. Because of pressure on the sole of the foot, the wart is pushed inward and a layer of hard skin may form over the wart. A plantar wart may be painful. Warts may spread through autoinoculation, by infecting nearby skin or by infecting walking surfaces. They may fuse or develop into clusters called mosaic warts.