Viroids are small, circular single-stranded RNA molecules. They are the smallest known pathogens, ranging in size from the Coconut cadang-cadang viroid (246 nucleotides) to the Citrus exocortis viroid (375 nucleotides).

Unlike a virus, the extracellular form of a viroid does not have a proteinaceous coat or capsid, it is naked RNA. Even more startling the RNA molecule does not contain code for any protein. The viroid is totally dependent on its host for replication.

Viroids are sometimes considered to be escaped introns and, like self-splicing introns, appear to be remnants of the RNA world.


Biology of Microorganisms. Brock et al. 7th Edition. Prentice Hall International, 1994