Define virus, virion and viroid and explain the differences.

Virus : Any of various submicroscopic agents that infect living organisms, often causing disease, and that consist of a single or double strand of RNA or DNA surrounded by a protein coat. Unable to replicate without a host cell, viruses are typically not considered living organisms

Virion : The infectious particle that is designed for transmission of the nucleic acid genome among hosts or host cells. A virion is not the same as a virus. I define virus as a distinct biological entity with five different characteristics

Viroid: An infectious agent that consists solely of a single strand of RNA and causes disease in certain plants. Viroids lack the protein coat (known as a capsid) of viruses and are the smallest known infectious agents.