Syphilis patients are the only source of infection. Sexual contact transmission accounts for 95%. Syphilis spirochetes are found in large numbers on the surface of skin and mucous membrane damage, as well as in saliva, breast milk, semen and urine. Untreated patients are the most infectious within a year of infection, and become less and less infectious as the duration of the disease increases. Syphilis spirochetes can also invade through dry skin and intact mucous membranes. A few can be transmitted through close contact such as kissing and breastfeeding, but the syphilis spirochete must be attached to the contact site. Since syphilis spirochetes are anaerobic and do not survive easily in vitro and are extremely sensitive to dryness, indirect transmission through various artifacts is very unlikely. Blood transfusion can be transmitted to the recipient if the blood donor is a syphilis patient. Congenital syphilis is transmitted from a pregnant woman with syphilis to her fetus through the bloodstream of the placenta. Generally, in the first four months of pregnancy, due to the protective effect of the trophoblast, the syphilis spirochete cannot pass, so the fetus is not infected in the first four months of pregnancy, later the trophoblast atrophy, the syphilis spirochete can enter the fetus through the placenta and infect the fetus.