Carrier

In medicine, a carrier was an asymptomatic individual who carried an infectious disease and spread it on to others, but was themselves immune to its ill-effects.

In 2266, Spock stated that he could not return to the USS Enterprise, despite not being affected by the life prolongation project viruses, as he was a carrier and would spread it to others if allowed to go back to the ship. (TOS: "Miri")

Typhoid Mary was a classic example of a Human carrier of a virulent disease on Earth during the 20th century. Doctor Leonard McCoy, likened Dr. Sevrin to Typhoid Mary, as he was a carrier of the virus Synthococcus novae. (TOS: "The Way to Eden")

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