Under normal circumstances, kissing does not transmit HIV, syphilis, or hepatitis, and the amount of viruses in saliva is very low and is not usually transmitted through saliva. However, when both parties kiss with bleeding or wounds in the mouth, it is possible to transmit the infection through the blood in the mouth. AIDS, syphilis and hepatitis are transmitted mainly through blood, sexual contact and mother-to-child transmission. Because the pathogens of the above diseases are high and active in blood and body fluids, some bad habits such as sharing needles, syringes, razors or sex toys, and having unprotected sex can easily cause the transmission of pathogens through blood or body fluids. In addition, the mother can transmit HIV, syphilis and hepatitis to the fetus through the placenta and umbilical vein. During labor and delivery, the fetus can be infected with HIV, syphilis and hepatitis through the wounds caused by compression of the birth canal. Breast milk can also contain pathogens that can be transmitted during the mother’s breastfeeding process. It should be noted that although kissing does not spread AIDS, syphilis, or hepatitis, it can easily spread some infectious diseases of the respiratory and digestive tracts, such as influenza, mumps, and hepatitis A. The pathogens of these diseases are found in saliva. The pathogens of these diseases are high and active in saliva, and are easily transmitted through saliva and mucosal contact, leading to infection.