Viruses are often considered non-living entities because they do not exhibit the characteristics of living organisms when outside a host. Specifically, they:
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Lack Cellular Structure: Viruses are not made up of cells; they consist of genetic material (either DNA or RNA) surrounded by a protein coat (capsid) and, in some cases, an outer lipid envelope. They do not have the cellular machinery necessary for metabolism or reproduction.
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Cannot Metabolize: Viruses do not carry out metabolic processes on their own. They do not eat, grow, or generate energy, which are common characteristics of living organisms.
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Require a Host for Replication: Viruses cannot reproduce independently. They must infect a host cell and hijack the host's cellular machinery to replicate and produce new virus particles.
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Do Not Respond to Stimuli: While living organisms can respond to environmental changes, viruses do not have the ability to respond to stimuli or adapt outside of a host environment.
Because of these characteristics, viruses are often classified as a distinct entity, separate from the traditional classifications of living organisms.