Biology Regents Practice Exam 2025 - Free Practice Questions and Study Guide

A virus is best defined as a microscopic infectious agent that requires a host cell to replicate. This definition highlights the unique nature of viruses, as they cannot carry out metabolic processes or reproduce independently. Instead, viruses invade host cells and hijack the host's cellular machinery to produce new virus particles. This reliance on a host for reproduction distinguishes viruses from living organisms, which can reproduce on their own.

The structure of viruses also supports this definition; they typically consist of genetic material (either DNA or RNA) surrounded by a protein coat and sometimes an outer lipid envelope. Without a host cell, viruses are unable to replicate or produce infectious units. This characteristic is what places viruses in a different category from living organisms.

In addition, viruses are not classified as bacteria, which are single-celled organisms that can live independently and multiply on their own. Viruses lack the cellular structure and metabolic pathways seen in bacteria and multicellular organisms, further solidifying their classification as obligate intracellular parasites.