Read the text below.
ARE VIRUSES ALIVE?
For about 100 years, the scientific community has repeatedly changed its collective mind over what viruses are. First seen as poisons, then as lifeforms, then biological chemicals, viruses today are thought of as being ∈a gray area between living and nonliving: they cannot replicate on their own but can do so ∈ truly living cells and can also affect the behavior of their hosts profoundly. Finally, however, scientists are beginning to appreciate viruses as fundamental players ∈ the history of life.
To Be or Not to Be
The seemingly simple question of whether or not viruses are alive, which my students often ask, has probably defied a simple answer all these years because it raises a fundamental issue: What exactly defines “life?” A precise scientific definition of life is an elusive thing, but most observers would agree that life includes certain qualities ∈ addition to an ability to replicate. For example, a living entity is ∈a state bounded by birth and death. Living organisms also are thought to require a degree of biochemical autonomy, carrying on the metabolic activities that produce the molecules and energy needed to sustain the organism. This level of autonomy is essential to most definitions.
Viruses, however, parasitize essentially all biomolecular aspects of life. That is, they depend on the host cell for the raw materials and energy necessary for nucleic acid synthesis, protein synthesis, processing and transport, and all other biochemical activities that allow the virus to multiply and spread. One might then conclude that even though these processes come under viral direction, viruses are simply nonliving parasites of living metabolic systems.
(Source: scientificamerican.com)
According to the text, a clear definition of virus is difficult because
