Define a virus.

A virus is accurately defined as a microscopic infectious agent that cannot reproduce outside a host cell. Viruses are fundamentally different from living organisms because they lack the cellular structure and metabolic machinery necessary for independent reproduction and survival. Instead, they rely on invading the living cells of a host organism. Once inside the host cell, the virus commandeers the cell's machinery to replicate and produce new virus particles, leading to infection. This dependency on a host for reproduction highlights why viruses are categorized differently from bacteria or fungi and considered non-living entities when outside a host organism. Understanding this distinction is crucial for studying virology and their role in diseases.