In 1977, a group of Italian researchers discovered a previously unknown antigen in the hepatocytes of patients with viral hepatitis B. It was assumed that this was the 4th antigen of the B virus (by analogy with the already known antigens HBs, HBc, HBe), and in connection with this it was named the 4th letter of the Greek alphabet - delta. Subsequently, experimental infection of chimpanzees with blood serum containing the delta antigen proved that this was a new virus. At the suggestion of the WHO, the causative agent of viral hepatitis D was named hepatitis delta virus - HDV.