Hepatitis B is a potentially life-threatening liver infection caused by the hepatitis B virus (HBV). The hepatitis B virus attacks the liver and can cause both acute and chronic disease that puts people at high risk of death from cirrhosis and liver cancer. The virus is most commonly transmitted from mother to child during birth and delivery, as well as through contact with blood or other body fluids. Hepatitis B virus (HBV) is a member of the hepadnavirus family. The virus particle (virion) consists of an outer lipid envelope and an icosahedral nucleocapsid core composed of core protein. The nucleocapsid encloses the viral DNA and a DNA polymerase that has reverse transcriptase activity. The outer envelope contains embedded proteins that are involved in viral binding of, and entry into, susceptible cells. The surface antigens (HBsAg) decorating the lipid envelope of HBV is produced in excess during the life cycle of the virus, and its presence in blood indicates current hepatitis B infection.