Why are viruses classified as obligate parasites?

Viruses are classified as obligate parasites because they lack the cellular machinery to replicate independently, relying instead on a living host cell for their reproduction. This means that a virus cannot reproduce on its own; it must enter a host cell and hijack the host's cellular processes to produce new virus particles. This dependence on a host cell for replication is what defines them as obligate parasites, as they must parasitize other living organisms to propagate and exist. While some viruses may cause harm to their hosts, this is not a defining characteristic of obligate parasitism, nor does it relate to their replication process. The ability to survive outside of hosts is also not a feature of all viruses, as many depend on specific environmental conditions to remain viable. Thus, the requirement for a living cell to replicate is the key aspect of why viruses are categorized as obligate parasites.