The short answer to why viruses are referred to as obligate parasites is that they cannot reproduce outside their host. Some authorities employ the word ‘grow’ for reproduce. Both terms bear the limitations of both English and current scientific understanding. Viruses don’t have reproductive systems and so don’t, in the strictest sense of the word, ‘reproduce’ – their host does. It is for this reason that viruses are often not considered to be living.
Viruses don’t have bodies, outside of a protective shell, called the capsid, (Lifescience, 2009) so they have no body to grow (they are acellular). If by grow it is meant that their population grows then yes it certainly can.
Virus population growth is accomplished by hijacking host (usually specific host) cell resources; everything from its dna/rna reproductive machinery to the host’s body (think legs for transporting virus to new populations, the sneeze to facilitate spread, to the host’s behavior as in rabies).
Another view is that viruses are obligate parasites since they cannot be artificially cultured. It seems that the preferred reasoning for thus classifying viruses is the first given – that in nature they only reproduce with a host (Elio, 2009).
In A Genomic Analysis of the Archael System Ignicoccus hospitalis-Nanoarchaeum equitans (Poda, et. al., Genome Biol, 2008.) it is suggested that obligate parasites, whether viruses or not, will have genomes of less than 1 Mb. A problem arises with this definition in that Mycoplasmas, which can be artificially cultured, are included in this definition but not in the first. It is therefore felt, at this time, division of obligate parasites should not rest on genome size.
Interestingly, The Encyclopedia Britannica states that viruses are obligate parasites as they “…lack metabolic machinery of their own to generate energy or to synthesize proteins…”. This expands on the requirement in that the thermodynamic requirement for life is included.
Given these varied definitions and recent discoveries in the variances of life itself, GRAJ-1, the arsenic-based bacteria specifically, it might be better to assign a sliding scale to how ‘alive,’ how ‘parasitic,’ or how ‘obligate’ something is.
Genome Biol. 2008; 9(11): R158. Published online 2008 November 10. doi: 10.1186/gb-2008-9-11-r158. PMCID: PMC2614490 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2614490/ posted: 17 February 2009 11:03 am ET LiveScience Staff
Of Terms in Biology: Obligate Parasite. April 23, 2009. Elio http://schaechter.asmblog.org/schaechter/2009/04/of-terms-in-biology-obligate-parasite.html
“obligate parasite.” Encyclopædia Britannica. Encyclopædia Britannica Online. Encyclopædia Britannica, 2011. Web. 26 Jan. 2011. <http://www.britannica.com/EBchecked/topic/423842/obligate-parasite>.
NASA Finds New Life (Updated). Jan. 2011. http://gizmodo.com/5704158/nasa-finds-new-life
