Verruca (plantar wart) on the big toe, contracted through the use of communal showers at a swimming pool. This verruca was photographed after treatment by a chiropodist, some weeks after its appearance: treatment consisted of paring off the top layers of dead skin & applying an ointment containing formaldehyde. The black threadlike areas revealed after treatment are blood vessels. Verrucae (the medical name for all types of warts) are caused by a virus; the plantar types that grow on the soles of the feet are subject to pressure and are therefore painful.