Which of the following is a unique characteristic of viruses?

Viruses have a unique characteristic in that they require living cells for reproduction. Unlike other microorganisms, such as bacteria or fungi, viruses do not possess the machinery necessary for metabolism or reproduction on their own. They cannot replicate or produce energy without invading a host cell. Once inside a host cell, a virus hijacks the cell's replicative and metabolic processes to create new viral particles.

This reliance on living cells is a fundamental aspect of viral biology. It distinguishes viruses from other pathogens that can grow and reproduce independently in their environments. Thus, this requirement for host cells is a defining feature that is critical to understanding how viruses operate and propagate.