Infection with HIV causes AIDS, a serious illness that interferes with the immune system. People with AIDS lose the ability to fight infection and certain types of cancer. However, many people with HIV infection remain healthy for years before developing AIDS. Hepatitis G virus (also known as GB virus C [GBV-C]) is another virus that, like HIV, is transmitted through contact with the blood of a person who has the infection. Many people who inject illegal drugs, such as heroin, into their blood have GBV-C infection. Unlike HIV, GBV-C does not cause serious illness. In fact, studies show that people who have HIV infection progress to AIDS more slowly if they also have GBV-C infection (co-infection). We do not know how GBV-C slows HIV infection. Some researchers think that GBV-C might interact with the immune system in a beneficial way.