Someone or something that carries or transports people, goods, or signals; in medicine, a person or animal that has a disease germ without showing symptoms.
From 'carry' plus the agent suffix '-er', meaning 'one who carries'. The disease sense developed later, treating germs as hidden passengers.
An airline carrier, a phone carrier, and a disease carrier all share the same basic idea: something that secretly or openly transports. The word reminds you that movement can be obvious, like planes, or invisible, like viruses riding inside you.
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