Warts can spread via "active" virus material that is touched, itched, rubbed or passed via towels or other personal clothing.
People inadvertantly speard "common warts" or "flat warts" by itching them and then touching other parts of their hands or body.
It is important to either keep them covered (if possible while treating them) or conciously remember not to itch them. Also, it is a good practice to NOT share clothing or towels with others unless they have been washed.
Wart can cluster. Some warts cluster as the grow.
To determine the specific type of wart to to: wartsremedy.com