The primary characteristic of a virus is that it is an acellular infectious agent that cannot replicate independently. Viruses consist of nucleic acid (either DNA or RNA) encased in a protein coat, known as a capsid. They require a host cell to reproduce, as they lack the necessary cellular machinery for metabolic processes. Upon entering a host, viruses hijack the host's cellular mechanisms to produce new viral particles, often causing cell damage. This pathogenic nature is what impacts the health of both humans and other organisms, making viruses crucial subjects of study in medicine and biology.