What is a defining characteristic of a virus?

A defining characteristic of a virus is that it is a tiny, nonliving particle that invades cells. Unlike living organisms, viruses cannot carry out metabolic processes on their own, nor can they reproduce independently. Instead, they rely on a host cell to replicate and produce new virus particles. When a virus enters a host, it attaches to a host cell and injects its genetic material into the cell's machinery, hijacking the cell’s processes to reproduce itself.

This distinguishing feature highlights that viruses are fundamentally different from living organisms and other biological entities. They lack cellular structure and do not possess the characteristics that we typically associate with life, such as the ability to grow or reproduce outside of a host organism. Their nonliving status is a critical factor in understanding how they interact with living systems and contribute to infectious diseases.