Hepatitis D is a disease caused by a small circular RNA virus (Hepatitis delta virus); this virus is replication defective and therefore cannot propagate in the absence of another virus. In humans, it only occurs in the presence of hepatitis B infection. When this transmission occurs simultaneously, it is called coinfection. When the hepatitis D virus infects cells previously infected with hepatitis B, it is called superinfection.

Either superinfection or coinfection can seriously worsen the damage to the liver caused by hepatitis B, causing cirrhosis or hepatocellular carcinoma (liver cancer).