Final answer:
Hepatitis D (HDV) can only infect individuals already infected with Hepatitis B (HBV) since it requires HBV for replication. Vaccination against HBV also provides protection against HDV.
Step-by-step explanation:
The form of hepatitis virus that can only infect an individual who is already infected with another hepatitis virus is Hepatitis D (HDV). HDV is an enveloped ssRNA(-) that is circular and can only propagate in the presence of Hepatitis B (HBV). Therefore, HDV infections can only occur in patients already infected with HBV, which it requires for replication. Vaccination against HBV is also protective against HDV infection, as HDV is transmitted through contact with infected blood and relies on the HBV life cycle.