A 33-year-old man who regularly injects drugs and recently tested negative for HIV presents for evaluation of a skin abscess. He injects heroin daily and typically uses his own clean needles. During the evaluation, he reveals that approximately 24 hours ago, he shared a needle with another person who was not well known to him. Prior to this recent event, the last time that he had shared needles or injecting equipment was about 4 months ago.