A sensitive was a person who was able to see and connect with the reservoir of evil which existed on a higher plane. They were capable of being possessed and used by the evil around geographical locations metaphysically closest to the reservoir.
Dr Colin Dove was somewhat sensitive and wished to know what would happen if the reservoir of evil were to be allowed to enter the physical world at perihelion. To this end, he planned to use Daniel O'Kane (or "Patient Zero") and Peter Russell, the strongest sensitives that he knew of (with Peter being the very strongest). Although Daniel died, Patient One was still alive and seemed to be connected to Russell in the same way that Daniel had been.