There are more than 200,000 identified species of virus and many more waiting to be discovered. Scientists do not know the origin of all viruses, so they cannot be placed within the ‘tree of life’ as other organisms are. The classification of viruses is based on their; size, capsid morphology, type of nucleic acid, mode or replication, host organism and pathology. The International Committee on Taxonomy of Viruses (ICTV) began its classification of viruses in the 1970s. It continues to develop and refine this classification. The taxonomy system for viruses shares certain features of the taxonomic system for living organisms. However, instead of the sequence ‘kingdom, phylum, class, order, family, genus, species’, virus classification begins with the ‘order’. Each taxon (group), apart from species, has a specific suffix; order -virales, family -viridae, subfamily -virinae, genus -virus and species. The Baltimore classification of viruses places viruses into seven groups (I-VII), depending on their type of nucleic acid.
Viroids are smaller than viruses. They are infectious pathogens and consist only of short strands of circular, single-stranded RNA. They do not have protein coats. Viroids do not code for any proteins and they replicate using the enzyme RNA polymerase. The first viroids was discovered in 1971. Prions is a term that was first used in 1982 and means proteinaceous (made of protein) infectious particle. It is derived from the words protein and infection. During the 1960s research scientists discovered that infectious proteins cause forms of transmissible spongiform encephalopathy, such as scrapie in sheep, and kuru (laughing disease) and new variant Creutzfeldt-Jakob disease (vCJD) in humans. Prions are not living organisms but are misfoled proteins (proteins that are incorrectly folded and consequently misshapen). Prions are stable and not denatured by heat or digested by protease enzymes.