www.oed.comb. Pl. viruses. An infectious organism that is usu. submicroscopic, can multiply only inside certain living host cells (in many cases causing disease) and is now understood to be a non-cellular structure lacking any intrinsic metabolism and usually comprising a DNA or RNA core inside a protein coat (see also quot. 1977). Formerly referred to as filterable viruses, their first distinguishing characteristic being the ability to pass through filters that retained bacteria.