Viruses do not possess all of the qualities of living things and so don’t tend to be classified in the standard hierarchy of living organisms. Therefore, they are not part of any of the five kingdoms of life (Five).
Viruses are one of the major unsolved mysteries in biology (Five). They are not made up of cells, and therefore cannot be grouped with prokaryotes or eukaryotes. They are thought to be somewhere between living and non-living systems.
Viruses aren't living beings because they cannot reproduce independently, unlike fungi and bacteria etc. (ICTV). Additionally, viruses don't contain certain cell structures found in all genuine living creatures; they do have a nucleic acid core and a protein casing but they don’t have any organelles. Therefore, as they aren't classed as being fully alive, they don't fit into any of the five kingdoms. However, they need to be placed somewhere, and so scientists tend to classify them by what other viruses they're similar to and what they are able to infect. Viruses are occasionally deemed to be a kingdom of their own (Wayne).
Viruses are active and reproduce only when they are inside the host cell. Alone, they are entirely static. They have a genetic material characterised by either DNA or RNA, and enclosed by a protein casing. Viruses use the metabolic apparatus and materials of the host cell in order to reproduce.
It is due to these various idiosyncrasies that viruses do not fall into any of the five kingdoms of life. Nevertheless, the International Committee on Taxonomy of Viruses is putting together a separate classification system for viruses (ICTV).
References
ICTV Database. Retrieved from http://www.ictvdb.org/index.htm
Five Kingdom Classification. Tutor Vista. Retrieved from
http://www.tutorvista.com/content/biology/biology-iii/modern-classification/five-
kingdom-classification.php
Five Kingdoms System. Web Rollins Education. Retrieved from
http://web.rollins.edu/~jsiry/5KingdomSys.html
The Five Kingdoms of Life. Wayne’s World Trivia. Retrieved from
http://waynesword.palomar.edu/trfeb98.htm
