Vermin

Vermin, a centipede like creature, surrounded by pool of blood, and targeted by members of the League who have the task to crush them all.

In-Game Information
This information comes directly from the game, or from official supporting materials.

Description
A description of facts about this subject, with minimal speculation.


 * Vermin can be collected upon defeating a boss while having the Impurity rune equipped and either complete a successful coop session, or having a fellow League confederate summoned. This include the following NPC summon: Younger Madara Twin, Henryk, Yamamura, and Valtr.
 * Another way of acquiring vermin is killing the few red eyed old hunters found in the Hunter's nightmare, with or without the Impurity rune required. Those do not respawn after death.
 * The hunter receives the League Staff from Valtr after crushing one vermin, and will cause him to disappear, leaving him One Eyed helmet behind, after talking to him with five Vermin crushed.
 * Vermin can also be simply translated as "bug" or "insect", which is also the term used to refer to phantasms.
 * "Impurity" rune can be translated as "Stagnation" or "Stagnant pool". The description also reads that vermin are the source of "stagnation of man", name shared by the japanese alternative translation for the Human Dregs in Dark Souls 3.
 * The mention of "filth" in the Vermin description also contains the term for "kegare", a japanese idea of defilement of the mind or body. The term also appears in Valtr's dialogue.

Theories
Speculation and related theory documents, which should not be treated as fact.


 * Vermin is real and at least contributes to beasthood:

The League Staff description mentions the need to uphold an illusion, presumably because the Hunter needs the Impurity rune to acquire Vermin from defeated bosses, and Valtr says that the Hunter should not expect the world to understand what the League is doing. This can lead to believe that the Vermin is not real and is merely the result of the illusion. However, this approach would mean that Valtr and the descriptions related to vermin are lying or unaware, and it would not explain why a few Vermin can be gathered by killing red eyed old hunters in the Hunter's nightmare regardless of whether or not the Impurity rune is equipped. Moreover, there are few instances where centipede can be found directly inside of beasts, such as inside the mouth of silverbeasts, or against the headless bloodletting beast. Those could fall into the category of vermin, centipede like creatures causing stagnation. Furthermore, fire is effective against beast type enemies and some of them are even afraid of it. The description of Molotov Cocktail reads that it is used to "cleanse impurity", and Vermin happens to be the root of man's impurity. It is also probably no coincidence that the snake who grew up alongside the Madara twins was fed with beasts and filled with Vermin, or why Valtr, who much like the snake ate a beast, was the one who led to the discovery of the Impurity rune, found inside him. He was able to see vermin because he ate something particularily impure and, therefore, filled with vermin. Another clue to consider is the description of the Madman Hood, which reads that "truth often resembles madness", something that could very well apply to Yamamura, who peered straight into vermin, and perhaps to the the younger Madara twin as well, who discovered vermin in his beloved snake and could have murdered his brother due to the induced madness. Valtr also notes that the Hunter is unbroken even after crushing multiple vermin, but also notices that in the glint of the Hunter's eyes that they encountered vermin. In other words, the reality and existence of Vermin would be the truth that causes madness. All of those evidences point at Vermin being real and at the very least involved in beasthood.

It is also interesting to note that red eyed Old Hunters drop Vermin, and that in Sekiro, red eyes are the sign of strong and unfulfilled desires according to the descripiton of Red Lump, a motif that could very well apply to other instances of red eyes in From Software games. In this case, the old hunters, as well as the few red eyed scourge beasts and beast patients, could be yearning and behaving like beasts, who "seek the warmth of blood" according to the description of Clawmark rune. Vermin being the root of man's impurity and a source of beasthood would also be consistent with the aforementioned Clawmark description which describes ""Beast" as an horrific and unwelcome instinct deep within the heart of men", further confirmed by the description of Beast Claws which mentions the "beast within", Djura who claims that beasts are people, the Suspicious Beggar who says that we are all "rancid beasts", etc. This gives credits to Valtr's words on human's corruption, and explains why he says that nobody out there deserve to live and why the Hunter dead body will curdle with Vermin if they attack him like a beast would, as humans have an inner beastly instinct inside and therefore are naturally impure due to Vermin. The motif of the impurity of humans and life is a recurring theme in From Software games. In Dark Souls, for instance, hollows are essentially the equivalent of beasts in Bloodborne, and they are essentially walking corpses, with a rotten stench according to Dark Souls 2 Hollow Mask description, and are the most honest shape of man according to Yuria and Vendrick. Much like beasts, they are life starved according to Alluring Skull description, animated by their yearning for souls, with yearning being the true essence of life according to Miyazaki's comments on the Gaping Dragon and Ancient Dragon's dialogue.


 * Vermin is a source of poison and impure blood:

As mentioned, Vermin is at least involved in beasthood. The description of Antidote reads that it was used to treat Ashen Blood or, in other words, poison, the ailment that "eventually spreads the scourge of the beast". Poison and Vermin seem to be both connected to beasthood, and it would make sense to things they are related between them as well. This much is consistent with the trend in From Software games to associate insects with filth, rot, poison, impurities, death, etc. This association applies to Bloodborne as well, given the description of the Hintertomb Root Chalice, which reads that "The hintertombs are unceremonious homes to graves and death, cesspools of noxious snakes and insects". The serpents in Forbidden Woods, place where League Confederates are prominent, including the the Madara twins and people conducted research on beasts, are seemingly covered by ticks. Vermin being referred to as "bugs" or "insects" in the japanese further plays into this idea. Another trend in From Software game is that blood can turn to poison, as evidenced in Dark Souls 2 Manslayer, Dark Souls 3 Hollow Knife, etc. There again, Bloodborne probably abides to this idea seeing that poison is also referred to as "Ashen Blood" and that Chikage transformed mode, "imbrued with blood", causes Rapid Poison, much like Queen Yharnam's blood attacks. Speaking of which, Vilebloods' blood is described as particularily impure by Alfred and Annalise herself, and this could also be due to Vermin, root of man's impurity. If the Vermin is what allows the Bloodletting Beast stand even with its head cut, then it could also be what makes Annalise undying as well. This would be in line with the undying monks, Guardian Ape, and Corrupted Monk in Sekiro, who are also inhabited by centipedes and cannot be killed by normal means. Sources of Rapid Poison such as the Chikage, Queen Yharnam's blood attacks, and the centipede coming out of the headless Bloodletting Beast could also be due to their impure blood, which matures into a more immediate effective poison than the regualr Slow Poison. It would thematically fit for Vermin to be a source of poison too, since the japanese text reads that it causes stagnation, which evokes rot. Finally, it is probably no coincidence that the cut Queenly Blood item received from Annalise looks like blood infected by a snake's poison.

Reference

 * 1) Thanks to bloodborne-wiki for their hard work gathering lore notes, item descriptions and dialogue.
 * 2) To LastProtagonist for the various translations on Vermin, Impurity, Valtr's dialogue, and more.
 * 3) To Herbert West for the insights on Vermin, Valtr's dialogue, stagnation, pollution, Queenly Blood, and more.
 * 4) To Ashley for the screenshots and observation on Forbidden's woods snakes, Bloodletting Beast, and red eyed motif.