Final answer:
Viruses are not considered living because they do not have cells, cannot metabolize or maintain homeostasis, and require a host to reproduce. While they do contain DNA or RNA, they do not possess the full suite of cellular components necessary for life.
Step-by-step explanation:
When assessing whether a virus is a living thing, we look at the key characteristics of life. A single virus particle, or virion, is comprised of genetic material encased in a protein coat known as a capsid. Despite having organization and the ability to evolve, viruses lack most attributes that define living organisms. They are not cellular, do not perform metabolism, cannot maintain homeostasis, and are incapable of independent reproduction. They must hijack a host cell's machinery to replicate, which differentiates them from living entities that replicate autonomously.
Viruses do not belong to any of the three domains of life since they are not composed of cells. While viruses contain either DNA or RNA, they lack other cellular components such as a plasma membrane, cytoplasm, and ribosomes. Because of their inability to reproduce independently and lack of metabolic functions, the scientific consensus is that viruses are not living.