Viroid

Part of speech: noun

Definitions

1. A type of pathogen that consists only of a short strand of RNA and is known to induce diseases in host plants
2. An infectious agent made purely of RNA without any protein elements, responsible for various plant ailments
3. An extremely small RNA-based pathogen causing disease in plants without any protein coating

Etymology: Viroids are among the smallest infectious agents known, and their name is a deliberate pairing of two older words. "Virus" came into English from Latin, where it meant poison or slimy liquid, and the "-oid" ending comes from Greek "eidos", meaning form or shape, which gave English the suffix meaning "resembling" or "like". Put together, "viroid" means something like "virus-like". The suffix did real work here. Viroids are not viruses: they are short loops of single-stranded RNA with no protein coat, far simpler than the viruses they were named after. The name signals a resemblance, a plant pathogen that behaves like a virus but is built on different lines. Scientists coined the term in the 1970s to describe the agent of potato spindle tuber disease, which was the first of these to be identified.