Viruses are considered non-living because they lack cellular structure and cannot carry out metabolic processes on their own. While they can reproduce, they can only do so by hijacking a host cell’s machinery. Outside of a host, viruses remain inert and cannot grow or respond to stimuli. They do not possess the characteristics of life such as independent growth, metabolism, or homeostasis. Although they can undergo mutation and evolve, these changes occur only during replication inside a host cell. Their inability to perform vital functions independently classifies them as non-living entities.