While under the influence of alcohol, pot, cocaine etc., people tend to engage in activities that normally they would not. They tend to take chances more often, and are less likely to use condoms or use condoms correctly. My role here is to inform people about HIV and other STD's. People can reduce their risk at ALL times. High risk sex doesn't just happen.
People allow it to happen.
I know many, many people who don't engage in high risk activities, even when offered the opportunity. They have control over their own activities and lives. In regard to the question about testing positive. This is an average, so not all people will test positive by this point in time. This is as accurate as any test in medicine could ever be.