It is possible to contract syphilis by kissing a patient with syphilis, but the chance of infection is very low. Because syphilis is a chronic sexually transmitted disease, the course of the disease can be long, but it is not always contagious. In patients with untreated syphilis, the infection is highly contagious for 1-2 years, and becomes less and less contagious as the disease lengthens, with the patient essentially becoming non-contagious after four years of infection. Syphilis is a sexually transmitted disease caused by the syphilis spirochete, which has been confirmed to be present in skin damage, blood, semen, breast milk and saliva of patients. Because of the presence of syphilis spirochetes in the saliva, it cannot be absolutely ruled out that kissing does not transmit syphilis. According to the above analysis, if a patient has been sick for more than four years, he or she will not be infected by kissing, but if he or she kisses a patient who has been sick for less than four years, there is a chance of infection.