These are unusual infectious agents characterised by having a very small genome and in the case of prions, possibly no genome at all. If you are only interested in prions, click here.
Viroids are common plant pathogens which are a serious economic problem.
Group A viroids are clearly different to those described above . Additionally it is speculated that Group A viroids may replicate in chloroplasts whereas Group B viroids replicate in the nucleus and nucleolus. 3 enzymic activities are required for viroid replication, an RNA polymerase, an RNAse and an RNA ligase. Group A viroids probably replicate via a symmetric rolling circle mechanism, whereas Group B viroids probably use an asymmetric mechanism. RNA pol II is probably the enzyme which does this.