Hepatitis d is an academic and clinical research field focusing on the study of the hepatitis delta virus (HDV) and the severe liver disease it causes. This area investigates the virology of HDV, its unique dependence on hepatitis B virus (HBV) co-infection, the pathogenesis of associated liver damage, epidemiology, diagnostic methods, and the development of therapeutic and preventative strategies, highlighting its significance due to the accelerated progression to cirrhosis and hepatocellular carcinoma compared to HBV mono-infection.