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 , 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. ()

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. ()

External link 
 

Medical conditions