Hepatitis D virus infection is present worldwide and affects all age groups. Geographical distribution parallels that of hepatitis B, thus prevalence is highest in parts of Africa, South America, Romania, Russia, the Asia-Pacific region and the Mediterranean region including Southern Italy. Since the 1970's and 1980's, prevalence has declined significantly in Europe. Infection in the Asia-Pacific region includes high prevalence in Vietnam and certain of the pacific islands, notably Kiribati, Niue, Nauru and Western Samoa. Infection is rare in Fiji and in NZ Maori.
Hepatitis D virus (HDV) is a replication defective ssRNA virus. It has an absolute requirement for hepatitis B virus (HBV) helper function for the production of infectious virus. To be infectious, the mature HDV must be encapsulated in an entirely HBV derived outer envelope consisting of all three forms of HBV surface antigen. The internal nucleocapsid structure is composed of the viral RNA genome plus about 60 copies of delta antigen, the only HDV-encoded protein.