What is the difference between a virus and a viroid?
Question
The other day I read about viroids and realized I had always lumped them together with viruses. Turns out, viroids are even simpler — just a naked RNA strand without any protein shell. This answer clears up all the key differences between viruses and viroids in a way that actually makes sense if you’re curious about how infections work.
Answer ( 1 )
What they are made of
Genome size
Viruses range from about 3 kb to over 1 Mb. Viroids are only 246‑401 nt – the smallest infectious agents we know.
Hosts
Viruses infect animals, plants, fungi and even bacteria. Viroids so far infect plants only.
Replication strategy
Viruses hijack the host ribosomes to make viral proteins and copy their genome using viral or host polymerases. Viroids are copied by the host RNA polymerase II via a rolling‑circle mechanism and never code for any protein.
Diseases
Virus examples: influenza, HIV, SARS‑CoV‑2, tobacco mosaic virus.
Viroid diseases: potato spindle tuber, citrus exocortis, coconut cadang‑cadang.
Detection
Viruses – PCR, antigen tests, electron microscopy.
Viroids – RT‑PCR, Northern blot or bioassay on indicator plants.
Resistance
Viroid RNA is surprisingly stable but lacks a protective capsid. Viruses rely on their capsid or envelope for environmental stability.
Bottom line
A virus is a protein‑wrapped genome that can code for proteins. A viroid is just a tiny RNA circle that parasitises the host transcription machinery without coding for anything. Thats the key difference.