Medical Definition of carrier
1a: a person, animal, or plant that harbors and transmits the causative agent of an infectious disease; especially : one who carries the causative agent systemically but is asymptomatic or immune to it <a carrier of typhoid fever>—compare reservoir 2, vector 2b: an individual possessing a specified gene and capable of transmitting it to offspring but not expressing or only weakly expressing its phenotype; especially : one that is heterozygous for a recessive factor
2: a usually inactive substance used in association with an active substance especially for aiding in the application of the active substance: asa: a support for a catalystb: a vehicle serving especially as a diluent (as for an insecticide or a drug)
3: a substance (as a catalyst) by whose agency some element or group is transferred from one compound to another
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