Describe the difference between a virus and a bacterium.

The distinction between a virus and a bacterium primarily revolves around their structure, reproduction, and biological characteristics. Viruses are classified as acellular entities, meaning they are not composed of cells and lack the machinery required for metabolism and self-replication. Instead, they require a host cell to replicate; they invade host cells and use the host's cellular machinery to reproduce and propagate.

In contrast, bacteria are unicellular organisms, possessing a complete cellular structure that includes all the necessary components for metabolism, growth, and reproduction. They can reproduce independently through processes like binary fission, enabling them to thrive in various environments without relying on a host.

This critical difference in cellular composition and reproductive strategy underscores why the correct answer highlights that viruses are acellular and depend on a host for replication, whereas bacteria are unicellular and can reproduce on their own.