Hepatitis B is an infection of the liver caused by the hepatitis B virus. HBV is transmitted by exposure to infectious blood or body fluids (e.g. saliva, semen, urine); forms of transmission include unprotected sexual activity, blood transfusion, mother-to-infant transmission, or consuming contaminated food/water. The acute illness causes liver inflammation, vomiting and jaundice, while chronic HBV infection often leads to liver cirrhosis and cancer. Roughly one third of the world's population have been infected with hepatitis B virus. 5-10% of adults and 90% of babies who have been infected will have the virus for the rest of their lives. The infection is preventable by vaccination. Diagnosis of chronic hepatitis B virus (HBV) infection has long been based on HBV serology and measurement of hepatocytic enzymes. With the development of therapies for chronic HBV infection, including interferon and lamivudine, quantitative detection of HBV has been used increasingly as the most important marker for monitoring HBV replication activity, disease progression, and assessing antiviral treatment.