How does a virus differ from a bacterium?

A virus is classified as acellular, meaning it does not have cellular structures like a bacterium, which is a single-celled organism. One of the defining characteristics of viruses is that they cannot replicate independently; they must invade a host cell and hijack the host's cellular machinery to reproduce. This dependence on a host cell for replication distinguishes viruses from bacteria, which are capable of binary fission and can grow and divide on their own in appropriate conditions.

The acellular nature of viruses highlights their unique biological status, setting them apart from bacteria, which are prokaryotic and considered living organisms. Because viruses lack the cellular components necessary for metabolic processes, they require a living host to provide those functions, making their replication entirely reliant on a host environment. This fundamental difference in replication and cellular structure is crucial for understanding the behavior, treatment, and impact of these two types of pathogens.