Plantar warts on my feet. Is this common? What is the cause?

Virus, very common. . The virus responsible for plantar warts is a very common one, and usually affects younger individuals (under 40-ish).
Caused by a virus. It is not uncommon and it is caused by a human papiloma virus in the outer layer of the skin of the sole there are 100 types of hpv and only a few are known to cause plantar warts.