Flat warts are caused by infection with the human papilloma virus. This virus is widely distributed, and there are many carriers in the population, and it can be spread through human-to-human transmission and self-transmission. 1. The human papilloma virus can be transmitted from one person to another: if the skin is broken and comes into contact with a person with flat warts, flat warts may occur, and it is common to see hands, forearms, face and other parts of the body that are exposed and easy to contact with other people. 2. self-transmission: usually the human body after the occurrence of flat warts, the virus can be through the hands of scratching, spread to other parts of their own, such as washing face, scratching the back of the hand caused by infection. In addition, it is important to note that if you have flat warts, you should actively seek medical attention to identify the cause of the disease and treatment.