The multivector pathogen (also agent or nano-agent) was an anti-Borg biological weapon developed by a Federation/Starfleet think tank that included former Borg drone Seven of Nine. The agent combined a retroviral delivery system for the hormonal formula previously developed by Doctor Beverly Crusher along with an experimental anti-assimilation nanite technology developed by Doctor Jarem Kaz and Commander Data during the Covington crisis, which also delivered a refined version of the Project Endgame virus.
When the Borg Collective launched a renewed attack in Federation space, assimilating the Einstein in late 2380, the USS Enterprise-E was summoned to Earth by Admiral Alynna Nechayev and briefed on the latest anti-Borg technology and given the multivector pathogen. The agent was later used by the crew to combat the Einstein.
Several months later, in January of 2381, the USS Bhutto encountered a Borg cube attacking the planet Barolia. Former Enterprise Chief of Security Zelik Leybenzon, now the Bhutto's security chief, recalled the multivector agent and suggested its use; as the Einstein drones were out of communication with the Collective, they had not yet adapted to it. Leybenzon and his volunteer security team were injected with the agent, and he carried an additional hypospray aboard the cube, hoping to infect the Borg there. However, instead of assimilating the team, the entire team was slaughtered without infecting the drones. While Leybenzon lay dying, he reached for the hypo, and instead a Borg drone scanned the technology; Zelik blamed himself for delivering it into their hands and dooming the Federation. (TNG novel: Greater Than the Sum)