In 1977, Mario Rizzetto and colleagues described a novel antigen in the nucleus of hepatocytes derived from patients infected with HBV. Antibodies against the so-called ‘delta antigen’ were detected in patients with a particularly severe course of HBv infection. Subsequently, the hepatitis D virus (HDV) was identified as the infectious agent causing hepatitis in the presence of HBv infection. Thus, hepatitis D can occur only in individuals who are also infected with HBV as HDV uses the hepatitis B surface antigen (HBsAg) as its envelope protein, which is essential for viral transmission. HDV infection can therefore occur as either a superinfection of chronic HBv infection or as simultaneous acute HBV and HDV coinfection.