Viruses constitute a large group of infectious organisms which are much smaller than bacteria. They possess a common characteristic - that of being obligate intracellular parasites for the cells of the hosts they select. This means that viruses can grow and reproduce only if they are part of larger living cells. For this reason, these organisms invade other cells and take control of their operations. As they do, the living cells they invade are destroyed and new viruses emerge. These new viruses then proceed to attack and encroach upon other cells, in the process spreading from one part of the body to another.
Going over these inherent characteristics of viruses, we proceed to repeat the question many people ask: "Are viruses living organisms?" To answer this question, we first have to make some concrete differentiation between living organisms and nonliving things. This can be done by pointing to a number of characteristics that living organisms possess, but which are absent in nonliving things.
Foremost among the characteristics possessed by living organisms is that they are highly organized; they have the capacity to organize the materials they are composed of. Living organisms have the capacity for gross physical movement, as well as the ability to respond to gross physical movement. All living organisms have the capacity for growth, in both size and number. Finally, living organisms possess the potentiality for reproduction and mutation.
Those who assert that viruses are nonliving things support their contention by pointing to certain nonliving things that exhibit some degree of organization, movement, reaction to stimuli, and growth. While this argument is quite convincing, there never has been any assertion that nonliving things can reproduce and mutate; what is known and generally accepted is that viruses replicate under suitable conditions.
By their mere possession of an extremely potent complement of genes, it can be said that viruses possess the true essence of life. Obviously, there is not much to gain from arguing this matter. On the other hand, much can be achieved if we recognize the fact that in viruses we are dealing with a system which may be intermediate between truly living organisms and truly nonliving things.
How then can we truly make an unambiguous and reasonable differentiation between living organisms and nonliving things? The answer probably lies in our being able to accept that viruses are molecules that differ from any other kind of molecules for the reason that they are able to undergo replication.
In their book, "Viruses and the Nature of Life" (Dutton, New York, 1961), the American biochemist and virologist Wendell Meredith Stanley and Evans G. Valens pointed out that "a virus comes to life the moment it infects a cell." This, according to the two, is perhaps the significant difference between viruses and other molecules.
1. MadSci Network: Virology, "Re: Are viruses living organisms. If they aren't how do they work?", by Dean Cliver, Faculty, Food Safety Unit, University of California, Davis - http://www.madsci.org/posts/archives/2001-04/988650714.Vi.r.html
2. Introduction to the Viruses - http://www.ucmp.berkeley.edu/alllife/virus.html
3. Cruz Lectures, "Characteristics of living organisms" - http://lisacruz2.tripod.com/id30.html
4. "Virus", from the Wikipedia - http://en.wikipedia.org/wiki/Virus