Virusoid

A virusoid is an infectious agent that infects plants in conjunction with an assistant virus; the assistant virus harbors the virusoid and is required for successful infection. Virusoids consist of a single molecule of single-stranded circular RNA that is several hundred nucleotides long and codes for nothing but its own structure.

In size and structure, virusoids are similar to viroids (circular RNA molecules that infect plants but don't require an assistant virus).

Virusoids, while being studied in virology, are not considered as viruses but as subviral particles. Since they depend on helper viruses, they are classified as satellites. In the virological taxonomy they appear as Satellites/Satellite nucleic acids/Subgroup 3: Circular satellite RNAs.

The term virusoid is also sometimes used more generally to refer to all satellites.

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