Hepatitis B causes significant morbidity and mortality worldwide. More than 400 million persons, including 1.25 million Americans, have chronic hepatitis B. In the United States, chronic hepatitis B virus infection is responsible for about 5,000 annual deaths from cirrhosis and hepatocellular carcinoma. Hepatitis B virus is found in body fluids and secretions; in developed countries, the virus is most commonly transmitted sexually or via intravenous drug use. Occupational exposure and perinatal transmission do occur but are rare in the United States. Effective vaccines for hepatitis B virus have been available since 1982; infant and childhood vaccination programs introduced in the 1990s have resulted in a marked decrease in new infections. Risk factors for progression to chronic infection include age at the time of infection and impaired immunity. From 15 to 30 percent of patients with acute hepatitis B infection progress to chronic infection. Medical therapies for chronic hepatitis B include interferon alfa-2b, lamivudine, and the nucleotide analog adefovir dipivoxil.