Why viroids and virusoids do not cause diseases in animals ?
Viroids mainly cause plant diseases but have recently been reported to cause a human disease.
The human pathogen hepatitis D is similar to viroids and is thought to be a viroid.
This disease was previously imputed to a defective virus called the delta agent. However, it now is known that the delta agent is a viroid enclosed in a hepatitis B virus capsid.
For hepatitis D to occur there must be simultaneous infection of a cell with both the hepatitis B virus and the hepatitis D viroid.
Virusoids are circular single-stranded RNAs dependent on plant viruses for replication and encapsulation.
Virusoids are similar to viroids in size, structure and means of replication.
Right now they only infect plants but further work may show that they do infect humans and cause human or animal diseases.