viroid

Viroids are small single-stranded, circular RNAs that are infectious pathogens. Unlike viruses, they have no protein coating. All known viroids are inhabitants of angiosperms (flowering plants), and most cause diseases, whose respective economic importance to humans varies widely. A 2023 metatranscriptomics study suggests that viroids and viroid-like elements can be found in all domains of life.

The first discoveries of viroids in the 1970s triggered the historically third major extension of the biosphere—to include smaller entities—after the discoveries in 1675 by Antonie van Leeuwenhoek (of the "subvisible" protozoa) and in 1892–1898 by Dmitri Iosifovich Ivanovsky and Martinus Beijerinck (of the "submicroscopic" viruses). The unique properties of viroids have been recognized by the International Committee on Taxonomy of Viruses, in creating a new order of subviral agents.