What defines a virus?

A virus is characterized as a tiny, nonliving particle that invades and reproduces inside a living cell. This definition captures the essence of a virus’s biological nature, emphasizing its reliance on host cells for replication. Unlike living organisms, viruses lack the cellular machinery necessary for independent reproduction and metabolism; they cannot carry out life processes on their own. Instead, they must enter a host cell, hijack that cell's machinery, and use it to replicate their genetic material and produce new virus particles.

This characteristic distinguishes viruses from living organisms, which can reproduce independently and perform metabolic functions autonomously. Additionally, the definition differentiates viruses from other pathogens like fungi or parasites. Fungi are considered living organisms that can also cause diseases but do not fit the criteria of viruses, as they have complex cellular structures and can thrive outside of host organisms. Similarly, large parasites, while capable of infecting both plants and animals, are living entities that can grow and reproduce independently, contrasting sharply with the nonliving status of viruses.