The transmission of hepadnaviruses between their natural hosts, humans, non-human primates, and birds, including intra-species host transmission and cross-species transmission, is a topic of study in virology. Hepadnaviruses are a family of viruses that can cause liver infections in humans and animals. They are Group VII viruses that possess double-stranded DNA genomes and replicate using reverse transcriptase. This unique replication strategy, combined with their extremely small genomes and a very narrow host and tissue tropism, has distinguished them enough to be classified in the family Hepadnaviridae. There are two recognized genera: Orthohepadnavirus, type species: hepatitis B virus (HBV) Avihepadnavirus, type species: duck hepatitis B virus (DHBV) Structure With the example of human HBV: the particular feature of the HBV structure is the presence of three different forms in the plasma of infected patients: Dane particle (diameter ≈ 42 nm): the complete virion, which is infectious and consists of an enveloped icosahedral nucleocapsid containing the viral genome, consisting of core protein and protecting the partially double-stranded DNA genome, bounding with DNA polymerase. The capsid is enveloped by a lipid bilayer that contains three forms of envelope proteins: small (S) proteins, intermediated (M) proteins, and large (L) proteins, and these proteins have different surface antigenes domains which contribute the viral infectivity: L protein (Pre S1, Pre S2, S), M protein (Pre S2, S), S protein (S). In figure 1, showing the simplified structure of HBV particles. Subviral sphere particles (diameter ≈ 22 nm), these smaller, non-infectious and are the most abundant particle in the blood of an infected one. They are assumed to have the ability of absorbing virus-neutralizing antibodies to facilitate the virus spread and maintenance in the host. Filaments (diameter ≈ 22 nm, length: 50 nm-70 nm), which are less known about, but they are actually consisted of many subviral sphere particles. Genome As with the example of HBV, showing in figure 2, four open reading frames are encoded (ORFs), all ORFs are in the same direction, defining the minus- and plus-strands. And the virus has four known genes, which encode the core protein, the virus polymerase, surface antigens (preS1, preS2, and S) and the X protein. The minus-strand DNA is complete and spans the entire genome, while the plus strand spans only about two-thirds of the genome length and have variable 3' ends. But for the avihepadnaviruses they normally extend plus-strands almost all the way to the modified 5'