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 to assimilate Humanity. Seven was rescued by Captain Janeway, however, before she could begin work on the charge. (VOY: "Dark Frontier")