Plantar warts aren't happy to stay put. Given the chance, they'll spread into clusters, grow to an inch-plus in circumference, and move to other parts of the foot. Scratching and touching the wart makes its spread more certain.
The good news is that while plantar warts like to move, they only like certain types of skin. They'll be happy to get all over the soles of your feet, but they'll stay away from other parts of your body. Plantar warts won't lead to genital warts, for example -- but they can make being on your feet very uncomfortable.