Viral sequencers were technological components of Borg nanoprobes, used in the process of infecting a victim of assimilation.
In 2375, the Borg Queen, who was holding Seven of Nine captive, assigned her the task of programming nanoprobes and enhancing their viral sequencers for use in a biogenic charge, with which she planned to attack Earth with in order to assimilate Humanity. Seven was rescued by Captain Janeway, however, before she could begin work on the charge. (VOY: "Dark Frontier")