Viruses are considered nonliving because they do not have the basic characteristics of living organisms. This includes the inability to reproduce on their own, the lack of cellular structure, and the inability to carry out metabolic processes. Without a host cell, viruses are unable to function or even survive. Additionally, viruses do not exhibit growth, responsiveness to stimuli, or the ability to maintain homeostasis. These characteristics are essential for defining something as a living organism, which is why viruses are considered nonliving.
