Hepatitis C virus (HCV) presents several challenges to the development of prevention programs. HCV infection is persistent in up to 80% of cases, and viremic individuals may transmit infection to others. With 65-90% of injection drug users anti-HCV positive, a large reservoir of infection exists in most drug-injector populations. Studying the genetic variability of HCV infections could permit researchers to reconstruct chains of viral transmission in IDUs. However, the relationship of HCV to HIV epidemiology remains unclear and may depend on whether the proportions of infectious persons in the population are similar for both viruses.