Goa'uld & Tok'ra Symbiotes
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Threshold: Fraiser's replication of Jaffa pouch conditions can sustain a healthy symbiote indefinitely. Summit: Tok'ra symbiote Lantash sustained for a year after brain-washed host Martouf was killed by Carter. Summit: Host Martouf became a Zatarc assassin.  Lantash carries his memories and feelings for Carter.
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Summit: dying or forcibly removed symbiotes now said to excrete a toxin which kills the host along with them. Summit: the Goa'uld population stagnates for the first time.  Warring System Lords embrace cannibalism. Summit: System Lords eating mature, sentient Goa'uld symbiotes with ceremony and considerable relish.