What is a virus primarily characterized by?

A virus is primarily characterized by its requirement to invade and reproduce within a living cell. Unlike bacteria or other microorganisms, viruses lack the cellular machinery needed for metabolism and replication on their own. They depend entirely on the host's cells to reproduce and propagate.

When a virus enters a host, it attaches to a specific cell type, injects its genetic material, and takes over the host's cellular machinery to produce more virus particles. This process not only highlights the parasitic nature of viruses but also differentiates them from other microorganisms that can reproduce independently and have their own metabolic pathways. Understanding this characteristic is essential for grasping how viral infections occur and why they can be so difficult to treat.