The study conducted at Kaiser Permanente in San Francisco involved more than 600 high-risk individuals including gay and bisexual men, as well as heterosexuals and injection-drug users. These individuals were healthy at the time of enrollment and were put on a daily regimen of a blue pill called Truvada as a pre-exposure prophylaxis (PrEP). In previous tests, the pill had been shown to prevent infection about 86 percent of participants.