Stargate SG-1/Jacob Carter
General Jacob Carter was an Air Force general. He suffered from cancer until he agreed to serve as a host for a Tok'ra symbiote, at which time he left the planet to serve as a member of the Tok'ra leadership. He was Lieutenant Colonel Samantha Carter's father.
General Jacob Carter, father of SG-1 team member Samantha Carter, had a long and distinguished career in the Air Force before he was diagnosed with cancer. He used some of the connections he had gained during his long Air Force career to attempt to get his daughter a position in NASA's astronaut program, thinking that her SGC cover story was true and she was actually working a mundane job at Cheyenne Mountain instead of fulfilling her childhood dreams of going to the stars. He was confused and angered when she turned him down. This confusion was short-lived, however.
As his cancer worsened, General Hammond filled him in on the work his daughter was doing as a member of SG-1, and he agreed to allow a Tok'ra symbiote whose host was dying of old age to infest him in order to cure his cancer and allow the symbiote to continue living. Jacob was "blended" with the ancient Tok'ra symbiote Selmak, and he became Earth's liason to the Tok'ra and helped Earth to form an alliance with the Tok'ra against the Goa'uld. Jacob/Selmak stayed with the Tok'ra for years, even after the Tok'ra broke off diplomatic ties with Earth, and the Tok'ra began to wonder if Selmak wasn't being influenced by Jacob's humanity to be sympathetic to the humans.
Selmak's old age eventually caught up with it. Selmak could have preserved Jacob's life when it died, but Jacob refused to let it because he believed that Selmak's experience would be of use in the fight against the Replicators. Selmak slipped into a coma, and was thus unable to save Jacob's life - when Selmak finally died Jacob went too.