What is a virus?

A virus is defined as an infectious agent that can replicate only within the cells of a living host. This means that viruses cannot reproduce or carry out metabolic processes independently; they require a host cell to provide the necessary machinery to make copies of themselves. When a virus infects a host, it introduces its genetic material into the host's cells, hijacking the cell's reproductive processes to produce new virus particles.

This characteristic distinguishes viruses from living organisms, which can reproduce on their own, as well as from bacteria, which are single-celled organisms capable of independent life and reproduction. Additionally, while some parasites may affect animals, they often have more complex life cycles and do not fit the strict definition of a virus. Thus, option B accurately captures the unique nature of viruses in the context of infectious agents.