Questions that there must be a good answer to, but I don't know what it is

AIDS vaccine in final testing

"Twenty years after HIV geneticist Bette Korber first began tackling HIV, her hard work—some would say "obsession"—may be finally paying off as she and her team gear up for the first round of human trials of an HIV vaccine."

This may sound like trolling, but it's not. How exactly does an AIDS vaccine trial work? My ignorant non-scientist's understanding is that a vaccine is what you take before you have the disease in order to not get the disease.  So how does that operate here? The instinctive answer would be 'We give a bunch of people the vaccine, then inject them with HIV-positive blood and hope for the best!'. Obviously this isn't what happens. But what does  happen then? How exactly do you do the controlled experiment for the question of 'you can't get AIDS if you have this vaccine' without, you know, someone that's willing to try to get AIDS?

I can only think of one possible answer, but it's not pretty.