What key feature differentiates viroids from viruses?

Viroids are distinct from viruses primarily due to their unique structure and composition. Unlike viruses, which are typically composed of genetic material (either DNA or RNA) surrounded by a protective protein coat called a capsid, viroids lack this protein encapsulation entirely. Viroids are small, circular RNA molecules that do not possess any protein structure. This absence of a protein coat is a fundamental characteristic that sets viroids apart from viruses.

Viruses often have lipid envelopes, which aid in their infection processes, while viroids do not. The replication rate of viroids is not necessarily higher than that of viruses; the replication mechanisms are different, but not directly tied to speed. Additionally, viroids primarily infect plants, but some viruses can infect other types of organisms, so the ability to infect only plants is not a key differentiator. Therefore, the lack of protein encapsulation is the crucial feature that distinguishes viroids from viruses.