Hepatitis Delta Virus (HDV) infections always occur in association with hepatitis B virus (HBV) infections because of their dependence upon the HBV. The hepatitis delta virion is composed of an outer lipoprotein envelope made of the surface antigen of HBV (HBsAg) and an inner ribonucleoprotein structure in which the HDV genome resides. Due to the dependence of HDV on HBV, the presence of HBsAg is necessary for the diagnosis of HDV infection.

HDV causes both acute and chronic infections. Asymptomatic and non-progressive illnesses are seen in a minority of cases, but a severe and progressive route to cirrhosis is present in most. Available data suggest that approximately 5% of HBV carriers worldwide may be infected with HDV; it is estimated that there are 18-20 million people chronically infected with HDV worldwide. However, the epidemiologic distribution of HDV infection does not parallel that of HBV, because areas endemic for HBV may be almost HDV free. The level of HDV endemicity is partly related to the route of transmission