Viroids are strange infectious agents. Intriguing and very interesting!
These are nothing but a piece (often a small one) of circular single stranded RNA. These viroids are just 250 to 500 bases long. What makes it interesting is the fact that they like prions defy the concept of cellular organisation!
These RNAs replicate by Rolling circle model of replication.
They are capable of infecting cells and cause diseases.
Many plants diseases are now known to be caused by various viroids.
These are found to exibit catalytic activity as well. Catalyses reactions like forming or breaking covalent bonds.
The only (?) human disease caused by viroids is Hepatitis D...