A subviral particle that infects plants and consists of a small naked single-stranded circular RNA molecule. The virusoid is introduced to its host cell by an infecting plant virus, in which it is carried as a passenger. The virusoid nucleic acid is replicated by the cell’s RNA polymerase, in the form of a precursor comprising tandem repeats of the virusoid structure. These repeats are cut out and the ends joined together in a self-splicing mechanism, in which the RNA acts as a ribozyme. Compare viroid.