What are viruses, viroids and prions?

Viruses:

1. Viruses are non-cellular, microscopic infectious agents that are capable of replicating inside a host cell.
2. They cannot be either living organisms or non-living.
3. They are tiny and smaller in size (between 30-50 nm).
4. 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
5. Examples: Influenza virus, mumps virus, rabies virus, poliovirus, Herpes virus, human immunodeficiency virus (HIV), etc.

Viroids:

1. Viroids were discovered by T.O. Diener in the year 1971.
2. Viroids are infectious particles composed of free, single-stranded circular RNA as their genetic material and lack a protein coat.
3. Viroids are smaller in size than viruses.
4. Infect only on plant cells.
5. Examples: Potato spindle, tuber viroid, Avsun viroidae, etc.

Prions:

1. Prions are said to be virus-like organisms made up of only a prion protein.
2. They have elongated fibrils that are said to be aggregations of the protein that makes them infectious prion.
3. Prions attack nerve cell fibers producing neurodegenerative brain disease.
4. This leads to "Mad cow" disease and also Creutzfeldt-Jakob disease in humans.