Any of various small naked single-stranded RNA molecules that infect plant cells and cause disease. Smaller than viruses, the 30 or so known viroids are not enclosed in a protein coat of any kind: they generally consist of fewer than 400 nucleotides, do not contain any genes, but do have ribozyme activity. The circular RNA strand undergoes extensive base pairing within itself, forming a double-stranded structure that mimics DNA and is replicated by the host cell’s enzymes. The progeny viroids spread to other cells, establishing a wider infection; in some cases they infect the seeds and hence the new plants following germination. Pathogenicity is thought to be due to RNA interference of the host’s genes induced by viroid-specific small interfering RNAs. Viroids include many commercially important disease agents, such as coconut cadang-cadang, citrus exocortis, and potato spindle tuber viroid. The agents are also spread by contaminated tools, insects, or rubbing by large animals. Compare virusoid.