as South and East Asia and sub-Saharan Africa. In some cases breast milk of HBsAg positive mothers has been found to be positive for the virus, but there have not been any reports of HBV transmission through breast-feeding, even before the availability of hepatitis B vaccine for infants. Horizontal transmission: through directly contacts with infected body fluids, like blood and lymph, in ways like sexual contacts, from health-care workers, or parenteral exposure; indirectly through transfusions and haemodialysis, and contaminated medical practices. Vertical transmission: through perinatal periods, mother to child. Cross-species transmission Cross-species transmission has not been proven yet, but if this occurs, the chance to eradicate HBV infection by immunization will be diminished due to the difficulty in controlling of natural virus reservoir. And increasing human encroachment on rainforest habitat and fragmentation of declining populations increases the interactions and consequently the risks of disease transmission between wild primates and human populations. Transmission of mammalian hepadnavirus strains between cross-species primate hosts has been found in many cases, recombination of hepadnavirus strains from cross-species primate hosts, further demonstrates that primate associated HBV strains can indeed share hosts in nature, and cross-species transmission of primate-associated HBV strains provides the probability of interspecies recombination. Within non-human mammals Gibbon HBV: Between gibbon and chimpanzee: the inoculation of GiHBV to a chimpanzee resulted in the acute hepatitis infection, and the virus isolated from the infected chimpanzee was GiHBV too. Woolly monkey HBV: Since black-handed spider monkey belongs to the same family and subfamily as the woolly monkey, it was chosen as a suitable small primate model to study WMHBV. The results showed that the black-handed spider monkey was susceptible to WMHBV, but developed only a subclinical infection. The susceptibility of chimpanzees to WMBHV was only limited. WHV: has only been detected in eastern woodchucks (Marmota monax), and does not infect related alpine marmosets. It infects woodchucks, but not other rodents like ground squirrels. But HDV with a WHV envelope (wHDV) has been shown to infect chimpanzees, and primary human hepatocytes. Vice versa a human enveloped hHDC could enter and (transiently) infect woodchuck hepatocytes in vivo without the need for a helper virus, but an infection of woodchucks or primary woodchuck hepatocytes with HBV has not been described. GSHV: infects woodchucks and ground squirrels, specifically some species of squirrel (Spermophilus beecheyi and Spermophilus richardsonii) and the phylogenetically closely related chipmunks. GSHV runs a milder course when inoculated