In 2377, Axum and other Borg drones in Unimatrix Zero designed a nanovirus that would mask their mutation, preventing the Collective from detecting and destroying them. As they were unable to act outside of Unimatriz Zero, Axum appealed to Seven of Nine and the USS Voyager for help.
Aboard Voyager, The Doctor modified the nanovirus to nullify the drones' cortical inhibitors, so that they will retain their memories and individuality outside of Unimatrix Zero and thus potentially resist the Borg from within. Captain Kathryn Janeway developed a plan wherein herself, Tuvok, and B'Elanna Torres would allow themselves to be assimilated, giving them access to the central plexus of a Borg tactical cube. Through the plexus, the nanovirus would be disseminated throughout the Collective instantly.
Torres succeeded in downloading the nanovirus into the central plexus, though the Borg Queen overwhelmed Tuvok's neural suppressant and learned of the nanovirus herself. She modified it to fatally degrade the autonomic nervous systems of the mutated drones, intending to distribute it through Unimatrix Zero. Before it could be deployed, Janeway ordered Voyager to destroy Unimatrix Zero by disrupting its neural interlink frequency. (VOY: "Unimatrix Zero", "Unimatrix Zero, Part II")