> The hepatitis D virus (also called delta virus) is a small circular RNA virus. Hepatitis D    virus (HDV) is found in nature only as a coinfection with HBV. The hepatitis D virus is    replication defective and therefore cannot propagate in the absence of another virus. > The presence of hepatitis B virus is required to provide the envelop protein, enabling    hepatitis D virus to spread from cell to cell, and to express its pathogenic potential.
> It requires the surface antigen of HBV for the encapsidation of its own genome.
> It is a small circular RNA virus which causes a liver disease hepatitis D.

Virus classification-
   Group: Group V ((-)ssRNA)
   Genus: Deltavirus
   Species: Hepatitis delta virus

Genome structure-
> The HDV genome exists as a negative sense, single-stranded, closed circular RNA,    with an envelope made up of HBAg. Nucleotide sequence is 70% self-complementary,    the HDV genome forms a partially double stranded RNA structure that is described as    rod-like. The virus core contains only one known protein, the delta antigen. HDV    produces two proteins called the small and large delta antigens (HDAg-S and HDAg-L,    respectively).
> HDAg-S is produced in the early stages of an infection and is required for viral    replication.HDAg-L, in contrast, is produced during the later stages of an infection,    acts as an inhibitor of viral replication, and is required for assembly of viral particles.

Viral replication-
> Replication occurs in the nucleus of the host cell via host RNA polymerase.Two    complementary strands of RNA are synthesized from the circular genome.One the    antigenome, is an exact compliment of genome,The second, a smaller fragment, is    polyadenylated and acts as the messenger RNA for the delta antigen. HDV do not    encode their own polymerase. Instead, replication of HDV requires a host polymerase    that can utilize RNA as a template.HDV relies on host cell machinery for replication,    and the viral genome (and antigenome) serves as ribozymes for self-ligation and    cleavage. Based on indirect evidence, RNA polymerase II has been implicated in the    replication of HDV.Normally RNA polymerase II utilizes DNA as a template and    produces mRNA.HDV indeed utilizes RNA polymerase II during replication. That is why    HDV is the only known pathogen capable of converting a DNA dependent polymerase    into an RNA dependent polymerase.