[4] A recent metatranscriptomics study suggests that the host diversity of viroids and viroid-like elements is broader than previously thought and may not be limited to plants, encompassing even prokaryotes.

[6] The first recognized viroid, the pathogenic agent of the potato spindle tuber disease, was discovered, initially molecularly characterized, and named by Theodor Otto Diener, a plant pathologist at the U.S. Department of Agriculture's Research Center in Beltsville, Maryland, in 1971.

[12] Diener initially hypothesized in 1989 that viroids may represent "living relics" from the widely assumed, ancient, and non-cellular RNA world, and others have followed this conjecture.

[70] This indicates that when viroids replicate via a double stranded intermediate RNA, they are targeted by a dicer enzyme and cleaved into siRNAs that are then loaded onto the RNA-induced silencing complex.

The category encompasses satellite RNAs (including small plant satRNAs "virusoids", fungal "ambivirus", and the much larger HDV-like Ribozyviria) and "retroviroids".

One of at least two open reading frames encodes a viral RNA-directed RNA polymerase, that firmly places "ambiviruses" into ribovirian kingdom Orthornavirae; a separate phylum Ambiviricota has been established since the 2023 ICTV Virus Taxonomy Release because of the unique features of encoding RNA-directed RNA polymerases but also having divergent ribozymes in various combinations in both sense and antisense orientation – the detection of circular forms in both sense orientations suggest that "ambiviruses" use rolling circle replication for propagation.

[73][74][75] "Retroviroids", more formally "retroviroid-like elements", are viroid-like circular RNA sequences that are also found with homologous copies in the DNA genome of the host.

[82] After applying metatranscriptomics – the computer-aided search for RNA sequences and their analysis – biologists reported in January 2024 the discovery of "obelisks", a new class of viroid-like elements, and "oblins", their related group of proteins, in the human microbiome.

[83][84] Diener's 1989 hypothesis[85] had proposed that the unique properties of viroids make them more plausible macromolecules than introns, or other RNAs considered in the past as possible "living relics" of a hypothetical, pre-cellular RNA world.

If so, viroids have assumed significance beyond plant virology for evolutionary theory, because their properties make them more plausible candidates than other RNAs to perform crucial steps in the evolution of life from inanimate matter (abiogenesis).

[15][16][17] However, recent studies suggest that the diversity of viroids and others viroid-like elements is broader than previously thought and that it would not be limited to plants, encompassing even the prokaryotes.

Despite numerous attempts over the years to isolate and purify the assumed virus, using increasingly sophisticated methods, these were unsuccessful when applied to extracts from potato spindle tuber disease-afflicted plants.

[86] In 2014, New York Times science writer Carl Zimmer published a popularized piece that mistakenly credited Flores et al. with the virioid - RNA world hypothesis' original conception.

[94] In January 2024, biologists reported the discovery of "obelisks", a new class of viroid-like elements, and "oblins", their related group of proteins, in the human microbiome.