Despite the similarities in names, all Hepatitis viruses belong to different virus families: Hepatitis A Virus is a Picornavirus, Hepatitis B Virus is a Hepadnavirus, Hepatitis C Virus is a Hepacivirus, Hepatitis D Virus is a Deltavirus and Hepatitis D Virus is a Hepevirus.
Viruses are extremely small in size. So tiny in fact that is very difficult to understand in what family a virus belongs. It is not surprising therefore that some viruses are unclassified or tentatively classified (because nobody has studied them in depth). Therefore, it will not come as a surprise that viruses appear to be in a wrong family or that an entirely new family has to be proposed.
In 1967, it was reported that a surgeon, named George Barker, was suffering from acute hepatitis. When serum from the patient was inoculated into a tamarin monkey (Saguinus spp.), it too developed hepatitis. In ‘honour’ of his being a patient, the virus was designated GB Virus, although has never been proven which specific virus infected Barker.
Of these, GB Virus-B, which is not a human pathogen, is only associated with hepatitis in tamarins, while GB Virus-A, GB Virus-C/Hepatitis G Virus and GB Virus-D in turn, infect primates, humans and frugivorous bats. Hepatitis C Virus and GB Virus-C/Hepatitis G Virus infect Old World primates, while GB Virus-A and GB Virus-B infect New World primates. Natural Hepatitis C Virus infections are limited to humans, although experimental infection of chimpanzees is well documented.
GB Virus-A, GB Virus-C/Hepatitis G Virus and GB Virus-D, unlike GB Virus-B, do not appear to be associated with any signs and symptoms of disease. Moreover, the liver does not appear to be the primary site of replication for GB Virus-A and GB Virus-C/Hepatitis G Virus. Instead, the lymphoid system seems to be their primary target.
Based on the structure of the GB Viruses, it was only recently proposed to classify GB Virus-A, GB Virus-C and GB Virus-D as members of a new fourth genus within the family Flaviviridae, named Pegivirus. The researchers also propose renaming 'GB' viruses within the tentative genus Pegivirus.
Simian Pegivirus (SPgV) and species-specific viruses would have the species name in subscripts. GBV-C (or HGV) would become the Human Pegivirus (HPgV), GBV-Ccpz would become SPgVcpz, and GBV-D would become Bat Pegivirus (BPgV). If BPgV isolates found in different bat species segregate by species as with SPgV, the host species will be identified as a subscript.
 Epstein et al: Identification of GBV-D, a novel GB-like flavivirus from old world frugivorous bats (Pteropus giganteus) in Bangladesh in PLoS Pathogens - 2010
 Stapleton et al: The GB viruses: a review and proposed classification of GBV-A, GBV-C (HGV), and GBV-D in genus Pegivirus within the family Flaviviridae in Journal of General Virology - 2011