Why is a virus referred to as poison?

Viruses occupy a special taxonomic position, they are neither plants, animals, or prokaryotic bacteria (single-cell organisms without defined nuclei), hence they are generally placed in their own kingdom. In fact, viruses should not even be considered organisms, in the strictest sense, because they are not free-living—i.e., they cannot reproduce and carry on their own metabolic processes without a host cell.

A virus is the infectious agent of small size and simple composition which can multiply only in living cells of a host (animals, plants, or bacteria). The name is from a Latin word meaning "slimy liquid" or 'poison'.