Scientists classify viruses as not living because they lack many of the characteristics that are considered essential for life. Although they contain genetic material (either DNA or RNA), they cannot reproduce on their own and need a host cell to replicate. They also do not have the ability to carry out metabolic processes, maintain homeostasis, or respond to stimuli. Additionally, viruses do not grow, maintain their own shape, or respond to external stimuli. As a result, they are classified as non-living entities that rely on host cells to perform essential life functions.
