The same thing that causes a wart anywhere: the HPV virus. There are many, many different strains of the HPV virus, and certain strains like to infect certain parts of the body, preferentially. Having said that, it’s not common for a plantar wart to infect skin other than the palms and soles. Remember that the area of the sole is called the "plantar surface" of the foot, and hence a wart on the sole will be called a "plantar wart". Common ways to become infected are to be in areas, barefoot, where others who have plantar warts have been: wet areas of a bathroom or pool area or sauna; gym floor. To help avoid getting a plantar wart, try to wear flip-flops all the time when in the wet areas.
The same thing that causes a wart anywhere: the HPV virus. There are many, many different strains of the HPV virus, and certain strains like to infect certain parts of the body, preferentially. Having said that, it’s not common for a plantar wart to infect skin other than the palms and soles. Remember that the area of the sole is called the "plantar surface" of the foot, and hence a wart on the sole will be called a "plantar wart". Common ways to become infected are to be in areas, barefoot, where others who have plantar warts have been: wet areas of a bathroom or pool area or sauna; gym floor. To help avoid getting a plantar wart, try to wear flip-flops all the time when in the wet areas.
It’s a virus.
As zitdr said. Just to add that the full name is Human Papilloma Virus (HPV)