An anonymous reader writes: A study published in the journal Clinical Infectious Diseases details the recent trial of a drug named Truvada, which researchers think might excel at preventing HIV infections (abstract). The scientists administered the drug to 657 people at high risk for contracting HIV, including users of injected drugs. At the end of the study, every single subject was still free of the virus. This is encouraging news in the fight against AIDS, though it shouldn't be taken to mean the drug is perfectly effective. Since researchers can't ethically expose people to HIV, we don't know for sure that any of the subjects were definitely saved by the drug. Other studies have also had to be stopped because it was clear subjects who were on a placebo were suffering from noticeably higher rates of infection. Leaders in the fight against AIDS say this new study closes a "critical gap" in existing research by demonstrating that Truvada can work in real-world health programs.