representatives of the families Pongidae and Hylobatidae. However, at present it remains to be elucidated if nonhuman primate HBV variants are transmittable to humans and whether they cause disease in humans. And till now, no report of HBV transmission from captive animals to humans currently exists. However the potential for zoonotic disease transmission exists where blood or body fluid exposure is common. Such scenarios could include chronically infected animals kept as family pets, close contact with caretakers, in situations in which chimpanzees are slaughtered and used as bushmeat. Avihepadnaviruses Within birds Many cases about avihepadnaviruses transmission between or within species have been studied. DHBV: infects only certain duck and goose species, but does not even infect Muscovy ducks or chickens, or infects them inefficiently. Heron HBV: does not infect ducks and only infects primary duck hepatocytes very inefficiently. Studies on different heron species which are geographically isolated populations suggest that lateral transmission, virus adaptation and environmental factors all play a role in HHBV spreading and evolution, but it is not clear whether HHBV infection occurs at all in free-living herons, due to the possibility of cross-species virus transmission. Crane HBV: cranes are phylogenetically very distant from ducks and are more closely to herons and stocks, but CHBV infects primary duck hepatocytes with efficiency similar to DHBV. Between humans and birds There is no evidence for transmission of avian hepadnavirus between avian and human, but recombination of avihepadnavirus strains between cross-species avian hosts can offer potentials for the transmission between human and bird, considering that many birds are one of the main sources of food for human. Factors affecting transmission Species specificity The special replication strategy gives this genus a special character, but also the high replication capacity and high error rate of the viral reverse transcriptase, thus HBVs from this genus have the ability to adapt to the host's environment, and varies from different hepadnaviruses, thus exhibiting species specificities. Proteins Pre S1 regions of the L protein, exhibiting different host specificity, if proteolytically being removed from HBV particles results in a loss of infectivity. The S protein therefore is needed, but not sufficient for HBV entry. In contrast to the M protein, the L protein is essential for the viral infectivity. These special proteins plays different role in different viruses, thus determining the different host ranges, if this were strictly true, we would expect that divergence in pre-S would affect