Hepatitis D primarily infects individuals who are already infected with which other virus?

Hepatitis D virus (HDV) is unique because it requires the presence of the Hepatitis B virus (HBV) to propagate. HDV cannot replicate and infect on its own; it is considered a "defective" virus. Infections with Hepatitis D typically occur in individuals who are already carriers of Hepatitis B, as HBV provides the necessary envelope proteins that HDV needs to form its viral structure. Co-infection with HBV and HDV can lead to more severe liver disease than infection with HBV alone, highlighting the dependence of HDV on HBV for its existence.

The other viruses mentioned, such as Hepatitis A, HIV, and Hepatitis C, do not interact in the same way with HDV. Hepatitis A is an entirely separate virus with no association with HDV, and although HIV and Hepatitis C can coexist with Hepatitis B, they do not have the same dependency on it that Hepatitis D does.