The causes of flat warts on the hands include viral infection, low body resistance, infection, trauma and so on. 1. Viral infection: flat warts are caused by HPV-3, 5, 8, 9, 10 and 11 (HPV is human papilloma virus). 2. low body resistance: the onset of flat warts and the body’s immune function state dysfunction is related, in the endocrine disorders, overwork, may cause the virus rapid reproduction, so as to form flat warts. 3. Infection: the source of flat warts is the patient and virus carriers, through direct or indirect contact transmission, such as sexual contact or use of the patient used towels. 4. trauma: trauma or skin breakage is an important factor in the occurrence of HPV infection. HPV has squamous epithelial cells, through the skin mucous membrane tiny breaks into the epidermis of the basal cell replication, proliferation, to the epithelial cells of the abnormal differentiation and hyperplasia, caused by the formation of flat warts.