- Hepatitis D virus (HDV) is a defective RNA virus that requires the presence of hepatitis B virus (HBV) to replicate.
- HDV uses the HBV surface antigen (HBsAg) to form its viral envelope, enabling it to infect liver cells.
- Without HBV co-infection, HDV cannot produce infectious viral particles or replicate, making it dependent on HBV for its life cycle.

Reference: Harrison's Principles of Internal Medicine, Volume 2, Chapter 173, page 1985