Medical Definition of bug
1a: an insect or other creeping or crawling invertebrate animal (as a spider)—not used technicallyb: any of various insects (as the bedbug or cockroach) commonly considered obnoxiousc: any of the order Hemiptera and especially of its suborder Heteroptera of insects that have sucking mouthparts, forewings thickened at the base, and that lack a pupal stage between the immature stages and the adult—called also true bug
2a: a microorganism (such as a bacterium or virus) especially when causing illness or disease <Enterococcus faecium, an especially nasty bug, often shrugs off even vancomycin, a last-resort antibiotic.—Avery Comarow, U.S. News & World Report, 4 Oct. 1999>b: an often unspecified or nonspecific sickness presumed to be caused by such a microorganism <a stomach bug> <suffering from the flu bug>
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