HIV cannot be completely ruled out after 20 days of high-risk behavior, because HIV is diagnosed by HIV antibody testing, but HIV antibodies do not appear immediately after the initial infection, there is a window period. The window period is about 2-4 weeks, so it takes 2-4 weeks to test for HIV antibodies, sometimes longer, so 20 days cannot completely rule out AIDS. If the HIV antibody is negative at 20 days, you should retest again after one month. If the test is still negative until three months later, AIDS can be basically ruled out. Blocking medication should be applied immediately after high-risk behavior, and is available at CDCs and hospitals specializing in HIV treatment in major cities. If it is not possible to guarantee the application within two hours, it should be applied within 24 hours and taken continuously for 28 days.