So I won't tell you to keep reading like others (which you should), but these studies don't work by some research scientist having an HIV+ blood IV at the ready with some "you might notice a little prick" trope, they offer these medications to people that are already in high-risk groups and are conducting activities that make them the most likely demographic to obtain or transmit HIV. Folks like promiscuous homosexuals, prostitutes, drug addicts (risk of sharing needles), and focus groups like that.