GB virus C (GBV-C) is a common virus that does not appear to cause disease. GBV-C persists in up to 50% of exposed individuals, and virus replication appears to be in lymphocytes including CD4+ T cells. GBV-C infection is common among HIV-positive people, and several studies have found that HIV-positive individuals co-infected with GBV-C survive for statistically significantly longer periods of time than people without GBV-C. In vitro studies suggest that GBV-C inhibits HIV replication and alters cytokine, chemokine and chemokine receptor expression. Thus, GBV-C may be a major factor influencing the natural history of HIV disease.