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, similar to other hepatitis viruses. Infection with HDV can lead to more severe liver disease compared to HBV alone, increasing the risk of cirrhosis and liver cancer. Vaccination against Hepatitis B is the most effective way to prevent HDV infection, as it prevents HBV infection, which is necessary for HDV to thrive.