virus - (virology) ultramicroscopic infectious agent that replicates itself only within cells of living hosts; many are pathogenic; a piece of nucleic acid (DNA or RNA) wrapped in a thin coat of protein

TMV, tobacco mosaic virus - the widely studied plant virus that causes tobacco mosaic; it was the first virus discovered (1892)

viroid, virusoid - the smallest of viruses; a plant virus with its RNA arranged in a circular chromosome without a protein coat

onion yellow-dwarf virus - the virus that produces stunting and yellowing of the leaves of onion plants

potato yellow-dwarf virus - the virus that produces stunting and yellowing of the leaves of potato plants