What is the definition of a virus?

The correct definition of a virus is that it is a tiny infectious agent that can only replicate inside the living cells of an organism. This characteristic is fundamental to understanding how viruses operate. Unlike living organisms, which can replicate independently, viruses lack the cellular machinery necessary for their reproduction. They must invade a host cell and hijack the host's biological processes to produce new virus particles. This replication cycle can lead to harm in the host organism and is responsible for the symptoms associated with viral infections.

Understanding this definition is crucial because it highlights the uniqueness of viruses in the biological world, differentiating them from other infectious agents like bacteria or parasites, which can often replicate and live autonomously.