Viruses:

Viruses depend on the host cells that they infect to reproduce. When found outside of host cells, viruses exist as a protein coat or capsid, sometimes enclosed within a membrane. The capsid encloses either DNA or RNA which codes for the virus elements. While in this form outside the cell, the virus is metabollically inert.When it comes into contact with a host cell, a virus can insert its genetic material into its host, literally taking over the host's functions.






Prions:

Prions are unprecedented infectious pathogens that cause a group of invariably fatal neurodegenerative diseases by an entirely novel mechanism. Prion diseases may present as genetic, infectious, or sporadic disorders. Prions a disease causing agent that is neither bacterial nor fungal nor viral and contains no genetic material.




Viroids:

Viroids are infectious agents composed exclusively of a single piece of circular single stranded RNA which has some double-stranded regions.Because of their simplified structures both prions and viroids are sometimes called subviral particles. Viroids mainly cause plant diseases but have recently been reported to cause a human disease