Medical Definition of bug
1a: an insect or other creeping or crawling invertebrate animal (as a spider)—not used technicallyb: any of various insects commonly considered especially obnoxious: as: bedbug: cockroach: head lousec: 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 disease-producing microorganism and especially a germb: a disease caused by such microorganisms; especially : any of various respiratory conditions (as influenza or grippe) of virus origin
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