Viroids are much smaller than viruses and are also considerably simpler, for they consist of no more than a single strand of RNA.
The RNA is not enclosed in any structure, and except during infection is not associated with any other chemical substances.
The typical viroid is an RNA molecule about 50 nm in length. Though at the present time viroids are only suspected of being the agents of certain diseases in animal cells, they are known to be the cause of a number of plant diseases including spindle tuber disease in potatoes.
Generally, infectious agents are either viroids, viruses, prokaryotic cells (e.g., bacteria), or eukaryotic cells (e.g., certain protists). What these infectious agents have in common is that their identity is defined by the nucleic acid that each carries. Although there is still considerable debate on the matter, it now appears that there may be an exception to this rule.
Scrapie (a disease of goats and sheep) and a disease of the nervous system in humans called Creutzfeldt-Jakob disease) appear to be caused by agents consisting only of protein; the agents of these diseases are called prions. The protein comprising a prion has a molecular weight between 50,000 and 100,000, corresponding to a particle size that is 100 times smaller than the smallest viruses.