A 13-year-old girl has two non-tender lesions on her fingers that have appeared over the past 5 months. On physical examination, there are 0.5-cm slightly raised, pebbly-surfaced, gray-white papules, one on the dorsum of her distal right index finger, and another periungual to her little finger. They gradually disappear over the next 18 months. Which of the following is the most likely factor in the pathogenesis of her lesions?