You get plantar warts like you get the flu -- from a virus. In the case of warts, it's human papillomavirus (HPV). There are more than 100 strains of HPV. Some cause cervical cancer, some cause genital warts, and some cause plantar warts.
To get into your system, the virus needs a break in the skin and weak antibodies that will fail to kill it on contact. Once inside, the virus makes skin cells multiply rapidly so that there is a thick growth of skin.
HPV prefers a warm, moist environment, such as a locker room or shower floor. (That's why college kids invest in shower shoes or flip flops for dorm bathrooms.) HPV can incubate up to 20 months, so it may be difficult to identify when the infection occurred. If you were barefoot around a pool, though, that's a good bet.