What defines a virus in the context of microorganisms?

A virus is defined as a microscopic infectious agent that can only replicate inside the living cells of an organism. Unlike larger single-celled organisms, such as bacteria, viruses do not have a cellular structure and cannot carry out metabolic processes on their own. They are distinct from other microorganisms because they hijack the host's cellular machinery to reproduce, which often leads to disease.

The option describing a group of microorganisms accurately captures the essence of what a virus is, particularly within the context of how some viruses are indeed pathogenic and can cause various diseases in humans, animals, and plants. Understanding viruses as a subset of microorganisms helps clarify their unique position in the microbial world, especially given that their modes of transmission, symptoms they cause, and treatment options often differ significantly from other types of microorganisms like bacteria or fungi.