Final answer:
Viruses are considered nonliving because they do not have cellular structures, cannot carry out metabolism or maintain homeostasis, and cannot reproduce without a host cell. They do contain genetic material and can evolve, which has sparked some debate about their classification.
Step-by-step explanation:
Viruses are often considered nonliving entities because they lack several fundamental characteristics that define living organisms. They do not possess cellular structures such as a plasma membrane, cytoplasm, and ribosomes, essential for carrying out life processes.
This lack of cellular structure means that viruses cannot carry out metabolism, maintain homeostasis, or grow on their own. Consequently, viruses are entirely dependent on their host cells to reproduce, a process known as replication.
While viruses do contain genetic material in the form of DNA or RNA, they cannot replicate without invading a host cell and using its replication machinery. This unique relationship makes them parasitic by nature. Despite not being classified as living, viruses evolve and adapt to their environment, which is a trait associated with living entities.
This paradox leads to debates about their classification. The traditional perspective remains that viruses, lacking most traits of life, cannot be considered alive. Four examples of human diseases caused by viruses are the flu (influenza), HIV/AIDS, the common cold (Rhinovirus infection), and COVID-19 (SARS-CoV-2).