Pathogen

Pathogen was a microorganism that carried a disease.

At some point in the 2020s, Adam Lang's bloodstream had been infected with an alien pathogen. It was later identified as Praxeus. (TV: Praxeus ^[+]Loading...["Praxeus (TV story)"])

When the Doctor explained Christmas to him, Davros compared the holiday to a pathogen, one carried in the hearts and minds of those infected. (PROSE: Father of the Daleks ^[+]Loading...["Father of the Daleks (short story)"])