The hepatitis D virus (HDV) is a small, defective virus that can only infect individuals who are already infected with the hepatitis B virus (HBV). It relies on HBV for its replication and spread, making co-infection possible. HDV is primarily transmitted through contact with infected blood, sexual contact, or from mother to child during childbirth. Infection with HDV can lead to more severe liver disease compared to HBV alone, increasing the risk of cirrhosis and liver cancer. Vaccination against HBV can prevent HDV infection, as the presence of the hepatitis B vaccine protects against the co-infection.