Viroids differ from viruses primarily in that viroids are much smaller and consist solely of a short strand of circular, single-stranded RNA without any protein coat, whereas viruses have either DNA or RNA enclosed in a protective protein coat called a capsid. Additionally, viroids infect only plants, while viruses can infect a wide range of organisms including plants, animals, and bacteria. Viroids lack the ability to encode proteins and cause disease through RNA-based mechanisms, while viruses encode proteins and use more complex infection mechanisms.

Key Differences Between Viroids and Viruses

• Structure: Viroids are just naked RNA molecules without a protein coat. Viruses have a protein capsid and sometimes a lipid envelope surrounding their DNA or RNA.

• Genetic Material: Viroids contain only single-stranded RNA. Viruses can have either DNA or RNA, single or double-stranded.

• Size: Viroids are smaller than viruses, often considered the smallest infectious agents.

• Host Range: Viroids infect only plants, causing diseases like potato spindle tuber disease. Viruses infect plants, animals, and bacteria.

• Replication and Pathogenicity: Viroids replicate via a rolling-circle mechanism and do not encode proteins; their pathogenicity is linked to RNA sequences. Viruses have more complex life cycles involving protein synthesis.

This distinction highlights that viroids are sub-viral agents simpler than viruses and unique in their biology and host range.