Common warts and flat warts are caused by infections of the same virus, mainly due to Human Papillomavirus (HPV), but not by the same type. There are over one hundred types of HPV, with more than eighty types associated with human diseases. The types of HPV that cause flat warts are mainly type 3 and type 5, while the types that cause common warts are mainly HPV type 1, type 2, type 4, and type 7. Therefore, common warts and flat warts are caused by different types of HPV infections. The appearance of flat warts and common warts is primarily due to minor lesions in the skin and mucous membranes, allowing the virus to enter the epithelial cells of the human body. There, the virus replicates and proliferates, leading to abnormal proliferation and differentiation of epithelial cells, forming benign growths on the epidermis.