Viruses are generally not regarded as living organisms as they lack the cellular machinery required to survive on their own. This distinction is somewhat tenuous as viruses carry their own unique genetic material and are capable of evolving, but the commonly accepted definition of "life" requires the presence of a metabolism.Continue Reading
To be considered alive, an organism must demonstrate a capability for metabolism, growth, reaction to external stimuli and reproduction with heredity. Biologists tend to reject the idea of living viruses because a viral organism fails to meet several of these criteria. Viruses have no metabolism of their own, for example, and must rely exclusively on the cellular machinery of their host to assemble the proteins they need to thrive.
Viruses also arguably fail at stimulus response. While it is true that viruses interact with their environment and that they must be able to interact with the membrane of a cell in such a way as to gain entry, these actions are effected entirely by the chemical properties of their component molecules. Viruses are passive entities that do not change state in any way as a response to the environment.
The distinction between "living" and "nonliving" is artificial, however, and the categories devised by human biologists are subject to alterations that could change the way viruses are regarded without necessarily changing the viruses themselves.Learn more about Biology