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VIRION
The virion represents a virus in its extracellular phase of the host, whereas virus is a broad general term for any aspect of the infectious agent which can act as an obligate intracellular parasite.
The difference between virus and virion is that, viruses are intracellular obligatory parasites whereas virions reside extracellular.
To ensure survival of a virus, the virion must fulfil two roles.
Protecting the genome from environmental damage (heat, desiccation, chemicals).
Facilitating the passage of the virus to the next host.
VIROIDS
In 1971, Diener, discovered an acellular particle that he named a viroid, meaning “virus-like.”
Viroids is a smallest infectious consist of single stranded RNA without protein coat.
The molecular weight of viroids is in the range of 11500-13000 Daltons and they are capable of self-replication.
The first viroid discovered was found to cause potato tuber spindle diseases. The disease causes slower sprouting and various deformities in potato plants (Fig. 3).
The precise mechanism of viroid replication is yet to be explored.
VIRUSOIDS
Virusoids are sub-viral particles best described as non–self-replicating ssRNAs.
The key difference between viroid and virusoid is that viroid is a small infectious agent composed only of a single-stranded RNA while virusoid is a type of infectious circular single-stranded RNA which requires a helper virus in order to infect the host cells.
PRIONS
At one time, scientists believed that any infectious particle must contain DNA or RNA but in 1982, Stanley Prusiner reported that protein may act as infectious agent while studying the scrapie (a fatal, degenerative disease in sheep). He received the Nobel Prize in 1997 for his work on prions.
Prions are made of only proteins, designated as PrP (prion proteins) and have no nucleic acid.
The gene coding PrP are found in the normal host DNA. In humans the PrP genes are located on chromosome 20.
How PrP cause diseases is yet to be explored. It is believed that prions are normal protein that becomes folded incorrectly.
Prions shows a long incubation period, hence were once assigned the name slow viruses.
Some diseases caused by prions are mad cow disease, scrapie, chronic wasting disease, kuru and Creutzfeldt-Jakob disease