What is the primary difference between a virus and a bacterium?

The primary distinction between a virus and a bacterium is that a virus requires a host to replicate. Unlike bacteria, which are single-celled organisms that can thrive and reproduce independently in various environments, viruses lack the necessary cellular machinery for metabolism and reproduction. They must invade the cells of a living host to reproduce, using the host's cellular mechanisms to create new virus particles.

This dependency on a host is why a virus cannot be treated as a living organism in the traditional sense, as it cannot carry out life processes outside of a host. In contrast, bacteria can reproduce on their own in suitable conditions, and they possess the structures and processes that characterize living cells, including cell walls, cell membranes, and the ability to metabolize nutrients.