T O Diener discovered a

Viroids are small, circular, single stranded RNA molecules that are the smallest known pathogens. The extracellular form of the viroid is naked RNA, ie, there is no protein capsid of any kind. Even, the RNA molecule contains no protein encoding genes and therefore, the viroid is totally dependent on host function for its replication. No viroid diseases of animals are known and the precise mechanisms by which viroids cause plant diseases remain unclear.