The smallest known agents of infectious disease. Conventional viruses are made up of nucleic acid encapsulated in protein (capsid), whereas viroids are uniquely characterized by the absence of a capsid. In spite of their small size, viroid ribonucleic acids (RNAs) can replicate and produce characteristic disease syndromes when introduced into cells. Viroids thus far identified are associated with plants.
Nine different viroids have been described from widely separated geographical locations and from an assortment of herbaceous and woody plants. Viroid infections in some plant species produce profound disease symptoms ranging from stunting and leaf epinasty to plant death, whereas infections in other species produce few detectable symptoms compared to uninoculated control plants. Viroids generally have a restricted host range, although several viroids can infect the same hosts and cause similar symptoms in these hosts. Good controls are not available for diseases caused by these small infectious agents other than indexing procedures to provide viroid-free propagules. See Plant viruses and viroids, Virus