Hepatitis C virus (HCV) can cause severe liver disease. It is transmitted when an uninfected person is exposed to the blood of someone with the infection. Before it became possible to test donated blood for HCV, many persons became infected when receiving blood transfusions. Other ways to spread HCV include illicit drug use; exposure to blood in the workplace (such as occurs with health care workers, firefighters, and police officers); and such procedures as body piercing, tattooing, or dental work with nonsterile tools. However, the source of infection is unknown in many persons. Although some people with HCV infection become severely ill (they almost die or need liver transplants), many remain healthy and have no symptoms. Among persons who have HCV infection with no symptoms, it is unclear how many also have damage to the liver with no symptoms (“silent” liver damage). Knowing the extent to which silent liver damage exists in people with asymptomatic HCV infection will help determine how aggressive doctors need to be when examining healthy people for HCV infection.