Warts are caused by an infection with the human papilloma viris, which is also known as HPV. The virus causes keratin, a hard protein, is produced in excessive amounts and develos in the top layer of skin or epidermis. The extra keratin porduces the rough, hard texture of the wart.
While warts are not considered contagious, it can be spread through close skin-to-skin contact. This is more likely if the skin is wet or damaged.
Warts can also be transmitted indirectly from contaminated objects or surfaces such as swimming pools.