The human hepatitis B virus (HBV) is a small DNA virus that belongs to the family Hepadnaviridae . In the developing world, the most common route of infection is maternal (mother to infant) . The next most common is early horizontal transmission within the family. In the developed world, the disease is mainly transmitted via unprotected sexual contact, blood transfusion and injectable drug use. Eight genotypes (A-H) with distinct geographic distribution have been identified :
A and D are most common in Europe
B and C are most common in China and Southeast Asia.
HBV infection occurs primarily in hepatocytes and is non-cytopathic. Infection with HBV occurs in phases that can run the gamut from a silent, acute phase that is resolved by the immune system to a persistent chronic stage of infection that often requires life-long therapy. In chronic hepatitis B infection, it is the long-term viral presence that causes hepatic inflammation leading to acute and chronic liver dysfunction including acute hepatic failure, cirrhosis and hepatocellular carcinoma.