The Yssgaroth Taint, also simply the Taint, (PROSE: A Bloody (And Public) Domaine [+]Jayce Black, The Book of the Enemy (Faction Paradox, Obverse Books, 2018)., etc.) or the taint, (PROSE: The Book of the War [+]Lawrence Miles, et al., Faction Paradox novels (Mad Norwegian Press, 2002).) and otherwise known as the V Factor (PROSE: Vampire Science) or vampire virus, (PROSE: All Flesh is Grass) was the distinguishing characteristics that "made vampires vampires".
The V Factor involved a gene that was added to the victim's biodata during vampire conversion. The Eighth Doctor devised a method of purging a vampire of the V Factor. (PROSE: Vampire Science) The "taint" was so called due to being a manifestation of the original inter-dimensional Yssgaroth whose Great Vampires (PROSE: The Book of the War [+]Lawrence Miles, et al., Faction Paradox novels (Mad Norwegian Press, 2002).) fought the Time Lords in the Eternal War of their early history; while the true Great Vampires were driven out, (TV: State of Decay [+]Terrance Dicks, Doctor Who season 18 (BBC1, 1980).) such that "no pure matter from the “Spiral Yssgaroth” survive[ed]", Yssgaroth biomass proved capabel of "hybridis[ing] and corrupt[ing] by its very presence"; the infected, (PROSE: The Book of the War [+]Lawrence Miles, et al., Faction Paradox novels (Mad Norwegian Press, 2002).) "pale shadows of the real thing", (TV: State of Decay [+]Terrance Dicks, Doctor Who season 18 (BBC1, 1980).) were beings "pulled out of shape by the Yssgaroth’s substance, malformed and predatory", and took various forms from elegant and cultured to the beastly Mal'akh grotesques. According to The Book of the War, the Great Houses were unable to eradicate the Yssgaroth taint from the universe because the Yssgaroth had entered history precisely as it came into existence, embedding themselves into its structure on a conceptual level. (PROSE: "Yssgaroth" [+]Part of The Book of the War, Lawrence Miles, et al., Faction Paradox novels (Mad Norwegian Press, 2002).)