Neurolytic Pathogen

A Neurolytic Pathogen is an artificial virus that interferes with cognitive functions, apparently by inducing neurons to undergo lysis (cellular destruction by membrane rupture). The pathogen seems to have the ability to disrupt electrical signals generated by cybernetic lifeforms and to interfere with the communications between the organic components and the cybernetic ones.

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 this 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 at the Unicomplex.

Once the pathogen was introduced, massive chaos was brought to the Collective. The Queen lost contact with many starships and outposts, and even began to physically deteriorate. It allowed USS Voyager to destroy the transwarp hub's manifold system from inside a transwarp conduit. (VOY: "Endgame")