Approximately 10 percent of people with hepatitis C virus (HCV) have no identifiable source of infection. However, the usual list of risk factors is by no means complete.
A recent study in the Lancet revealed that HCV may have been transmitted at substantially high rates prior to 1965 because of medical reasons rather than injection drug use or other behavior-related causes (unsafe tattooing, high-risk sex practices, etc.). At that time, needles and syringes were reused; infection control was not nearly what it is today.
Although not knowing how you got hep C can be frustrating, this may be offset by the fact that hepatitis C infection is now curable. You may not know how you got the virus, but if you are cured, you will know how you got rid of it.