Virusoid

A virusoid is a circular, single-stranded RNA molecule dependent on a helper virus for replication and encapsidation. It is a type of subviral agent, distinct from viruses and viroids. Unlike viruses, virusoids lack the genes necessary for their own replication; they rely entirely on the replicative machinery of a helper virus co-infecting the same host cell. Similar to viroids, virusoids consist only of a small RNA genome without a protein coat (capsid). However, unlike viroids, virusoids are packaged inside the capsid of their helper virus. Therefore, for a virusoid to propagate, both it and its helper virus must successfully infect the host.

Virusoids are primarily known to infect plants. Their small RNA genomes typically contain a single gene or no genes at all. The RNA sequence often contains a conserved region that facilitates replication by the helper virus's RNA-dependent RNA polymerase. The presence of a virusoid can sometimes exacerbate the symptoms caused by the helper virus alone, leading to more severe disease in the infected plant. The classification and nomenclature of virusoids can be complex and are often related to the helper virus with which they are associated.