Viruses are non-cellular, microscopic infectious agents that are capable of replicating inside a host cell.
They cannot be either living organisms or non-living.
They are tiny and smaller in size (between 30-50 nm).
They generally lack a cell wall but are surrounded by a protective protein coating known as the capsid. It contains either RNA or DNA as the genetic material
Examples: Influenza virus, mumps virus, rabies virus, poliovirus, Herpes virus, human immunodeficiency virus (HIV), etc.
Viroids:
Viroids were discovered by T.O. Diener in the year 1971.
Viroids are infectious particles composed of free, single-stranded circular RNA as their genetic material and lack a protein coat.
Viroids are smaller in size than viruses.
Infect only on plant cells.
Examples: Potato spindle, tuber viroid, Avsun viroidae, etc.
Prions:
Prions are said to be virus-like organisms made up of only a prion protein.
They have elongated fibrils that are said to be aggregations of the protein that makes them infectious prion.