This is an open label, exploratory study of simtuzumab (GS-6624) in adults infected with HIV, hepatitis C virus (HCV), or co-HIV/HCV with histological evidence of liver fibrosis. Participants will receive simtuzumab 700 mg intravenously every 2 weeks for a total of 24 weeks (12 infusions) while continuing on standard therapy for HIV (HIV-infected subjects only).