A neurolytic pathogen is an artifical virus which intereferes with cognitive functions by inducing cellular neurons to self-destruct. Evidently also causing disruption of electrical signals generated by cybernetic lifeforms and interfering with communication between organic and cybernetic parts. The only known neurolytic pathogen was invented In an alternate timeline in the year 2404 by the future self of Voyager's EMH. The pathogen was first encountered by the Borg in 2378 when Admiral Janeway's counterpart from the year 2378 allowed herself to be infected and later flew in-to the heart of the collective - Unimatrix One. She allowed herself to be assimilated whilst still infected, essentially becomming a carrier for the most deadly anti-Borg weapon ever created. Introduction of the pathogen in-to the Collective caused almost immediate large-scale chaos, disrupting the Collectives communication to the Queen for all but one ship, followed by physical deterioration of her biological body and malfunctions in her bodysuit. Moments later large-scale explosions took place inside of Unimatrix One, it is not known if any portion of Unimatrix One survived this pathogenic attack.
Sphere 634 was however able to assimilate the neurolytic pathogen and the ablative armour technology which allowed it to maintain contact with the Queen and follow her instructions to persue the USS Voyager as it attempted to use the Transwarp Hub in sector ??? to return to Earth.