The viroids are the smallest pathogens. Are small molecules of single - stranded RNA and circulate without capsid. Its extracellular form consists of naked RNA. RNA takes on a secondary structure that looks like double-stranded DNA. Its RNA does not function as messenger RNA, nor does it contain information to translate it into enzymes that participate in its replication. Thus, viroids are totally dependent on host cell metabolism for replication.
They are the cause of important plant diseases, such as exocortis in citrus fruits or cadang-cadang in coconut.