The hepatitis D virus (or delta agent) is a defective virus whose replication has an absolute requirement for the hepatitis B virus. Type D hepatitis occurs exclusively in patients simultaneously or previously infected by the hepatitis B virus, often intravenous drug abusers or male homosexuals. Infection by the hepatitis D virus is often responsible for an increase in the severity of the hepatic lesions due to the hepatitis B virus. It can result in fulminant hepatitis. The best prevention is vaccination against hepatitis B.