Viroids are foreign, naked circular RNA molecules (nucleic acids) within a cell. Their multiplication takes place via cellular polymerases in the cell nucleus. They are mainly known as causative agents of plant diseases. Their significance for humans is controversial.
The hepatitis D virus shows similarities to a viroid. As with a viroid, the replication of its RNA does not depend on a helper virus, but on cellular polymerases. In order to develop into an infectious particle, HDV requires the envelope protein of the hepatitis B virus (HBs antigen). HDV infections therefore only occur in combination with HBV.