Is it ensure, insure, or assure?
late 14c., agent noun from carry (v.). Meaning "person or animal that carries and disseminates infection without suffering obvious disease" is from 1899; genetic sense is 1933. As a short form of aircraft carrier it dates from 1917. Carrier pigeon is from 1640s.
carrier car·ri·er (kār'ē-ər)
A person or an animal that shows no symptoms of a disease but harbors the infectious agent of that disease and is capable of transmitting it to others.
A compound capable of transferring a hydrogen atom from one compound to another.
A quantity of naturally occurring element added to a minute amount of pure isotope, especially a radioactive one, to facilitate the chemical handling of the isotope.
An individual that carries, but does not express, a gene for a particular recessive trait, yet when mated with another carrier, can produce offspring that do.