What is one common characteristic of all viruses?

Viruses are unique infectious agents that fundamentally differ from living organisms. One defining characteristic of all viruses is their requirement for living cells to replicate. Unlike bacteria or fungi, viruses cannot reproduce independently; they lack the cellular machinery necessary for metabolism and growth. When a virus infects a host cell, it hijacks the host's cellular machinery to reproduce, creating new virus particles. This dependency on a host cell for replication underscores the nature of viruses as obligate intracellular parasites.

The other options highlight aspects that do not apply to viruses. For instance, viruses cannot live outside a host because they are inert until they enter a living cell. They do not produce their own energy, as they lack the metabolic processes found in living organisms. Additionally, viruses are not made of living cells; instead, they consist of genetic material (either DNA or RNA) encased in a protein coat, and sometimes an envelope, but they do not possess the complex cellular structure characteristic of living cells.