The hepatitis D virus is a so-called parasite. This means that it cannot reproduce independently. It needs the shells of the hepatitis B virus to do so. Hepatitis D therefore always occurs simultaneously with hepatitis B. Either one is infected with both viruses at the same time or already carries the hepatitis B virus at the time of infection with hepatitis D.
Ways of transmission are the same for hepatitis D as for hepatitis B: contaminated blood, unprotected sexual intercourse, sharing needles among injecting drug users. An additional infection with hepatitis D often worsens the course of a hepatitis B infection, which increases the risk of liver cirrhosis or liver cancer. People with a chronic hepatitis B and D infection are advised to undergo antiviral therapy to minimise this risk. Vaccination against hepatitis B protects against hepatitis D as well.