Usually, the CDC draws blood mainly to check for infectious or contagious diseases, such as AIDS, syphilis, dengue fever and other diseases that can be detected. 1. AIDS: If the examinee has contact with people who are at high risk of AIDS, he/she can go to the CDC to have blood drawn for AIDS testing, because AIDS is usually diagnosed by detecting the presence of AIDS-specific antibodies or antigens in the examinee’s blood. 2. Syphilis: Syphilis is a disease caused by the syphilis spirochete, and after its invasion into the body, antibodies to the syphilis spirochete can appear in the blood, and such antibodies can exist for a lifetime, so if the results of the blood test show that specific antibodies to the syphilis spirochete are positive, it generally suggests that the patient is suffering from syphilis. 3. Dengue fever: dengue fever can usually be diagnosed by blood sampling, and if the antibody titer of the double serum of the examinee in the recovery phase is more than four times higher than that in the acute phase, the diagnosis of dengue fever can be confirmed. It should be noted that, in addition to the above diseases, blood sampling at the CDC can also be checked for influenza and other diseases, so if the examinee suspects that he or she is suffering from any of the above diseases, he or she should go to the CDC in time for the corresponding examination.