Final answer:
Viruses are not considered living because they lack important components of cells and cannot reproduce on their own.
Step-by-step explanation:
Viruses are not considered living because they lack the components shared by all cells, such as a plasma membrane, cytoplasm, and ribosomes. Although viruses contain genetic material (DNA or RNA), they cannot reproduce on their own. Instead, they infect host cells to make copies of their own genetic material.
Viruses also do not have their own metabolism or maintain homeostasis, which are essential characteristics of living things. Therefore, most scientists do not consider viruses to be living organisms.