A neurolytic pathogen is an artificial virus that interferes with cognitive functions.
In an alternate timeline, it was invented in 2392 by The Doctor.
In 2378, Admiral Kathryn Janeway, who had come back from the year 2404, infected herself with such a pathogen and allowed herself to be assimilated by the Borg Queen. In essence, Janeway became a carrier for a vastly destructive weapon that was taken directly to the heart of the Borg Collective.
Once the pathogen was introduced, massive chaos was brought to the Collective. The Queen lost contact with many ships and outposts, and even began to physically deteriorate. The spread of the pathogen led to the complete destruction of the Unicomplex. (VOY: "Endgame")
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