Hepatitis D (delta) is an 'incomplete' virus and can’t function on its own. A person cannot be infected with hepatitis D on its own, only as a co-infection with hepatitis B. The addition of hepatitis D can make chronic hepatitis B significantly worse and increases the risk of cirrhosis and liver cancer. Hepatitis D is the most dangerous known hepatitis virus.

Hepatitis B and D co-infection is relatively uncommon in western and central Europe with the exception of Italy. Migrants from South America, Africa, the Middle East and Eastern Europe, or people who contract hepatitis B in these regions, are more likely to have been exposed to hepatitis D.

Hepatitis D is mainly transmitted through blood-to-blood contact.