BioConcepts

viroidViroid (ger.)

• 1) An intracellular virus-like entity that functions as a symbiont. (OED 2012)
  category virus

  1946

  there exist ultra-microscopic organisms which are akin to viruses but which are useful symbionts, and […] these symbionts occur universally within the cells of larger organisms[:] viroids
  

  Altenburg, E. (1946). The “viroid” theory in relation to plasmogenes, viruses, cancer and plastids. Amer. Nat. 80, 559-567: 559.

• 2) A type of infectious agent of plants which is smaller than a virus and consists of circular single-stranded RNA without a protein coat. (OED 2012)
  category virus

  1971

  Unlike conventional viral nucleic acids […] none of these RNAs code for specific coat proteins that are capable of self-assembly into capsids. These entities, therefore, exist only in the vegetative phase; the dormant phase so characteristic of viruses (i.e., the virion) is absent. I propose the term ‘viroid’ for such entities. Altenburg (1946) introduced this term to designate hypothetical symbionts, akin to viruses […]. If, however, the ‘viroid’ is redefined operationally and in modern terms to encompass nucleic acid species with the properties discussed here, the term serves a useful function.

  Diener, T.O. (1971). Potato spindle tuber “virus”, IV. A replicating low molecular weight RNA. Virology 45, 411-428: 426; cf. Marx, J.L. (1972). “Viroids”: a new kind of pathogen? Science 178, 734.