These are unusual infectious agents characterised by having a very small genome and in the case of prions, possibly no genome at all.
Viroids are common plant pathogens which are a serious economic problem.
They are all single stranded covalent circles. There is extensive intramolecular base pairing. They replicate in the plant nucleolus. A DNA directed RNA polymerase makes both plus and minus strands. Replication does not depend on the presence of a helper virus.