We know that the abyss corrupt right? and in 1 youd need the covenant of Artorias ring to survive the abyss right?  However you didn't needed it in the dlc

 what if humans, did not needed Protection against the abyss after all the tomb of giants' darkness bares close resemblance to the Abyss... what if the ibly reason younneeded the ring was becayse the 4 kings ruked over the abyss in New Londo, thus making it aggressive... after all once they are gone, you do  not need the ring anymore...

 Furthermore, Pus-of-man bares similarities to Manus, so what if something allowed the abyss to corrupt said humanity, maybe by the process of hollowing? an healthy human with humanity couldn't be corrupted but what if once seperated, aka a mindless hollow without "humanity"  basically an empty shell, could get corrupted

as for the humanity itself, Dark Souls 3, Aldrich Faitfull covenant item Human Dreg looks like humanity so could it be theorized that humanity alone ( ie Humany Ghost) could be corruoted on the long term forming the Deep

the Deep gem description is this: " A gem of infused titanite. Found in the dregs of the Cathedral of the deep.

Used in infusion to deep weapons.

Deep weapons inflict dark damage, but lose scaling effects.

There is a darkness that lies beyond human ken." That darkness, what if the Darkness of man is what gets corrupted by the Abyss, woukd explain Manus... the human dreg description  is this : " Proof of a duty fulfilled by the Aldrich Faithful, who patiently await the Devourer of Gods' return.

Dregs are the heaviest things within the human body, and will sink to the lowest depths imaginable "   IRL, some people say that the soul is the heaviest thing, Imjust look at Egyptian mythology... there siuls was put on a scale   and if it was heavier than a feather they ould get devired by a saurian like God... Iudex transformation somewhat looks like a crocodile... Plus Human Dreg sprites looks like a deflated humanity sprite... the Deep soul spells says " Sorcery of Archdeacon Royce and his deacons, said to have been imparted to them by McDonnell of the Boreal Valley.

Fires dark soul dregs.

Souls which swell from the deep pursue their target, drawn towards life." Humanity is not mentioned yes however Humanity is never mentionned in Dark Souls 3, what uf Humanity and soul are one and the same for the people of that time ? Deep Souls stronger version, Great Deep Soul's description offers no further hint of this however its sprite start to look more like the humanity sprite from dark souls one, this brings us tonthe strongest version of Deep soul, Great Soul Dreg, notice how the name changes substentially, its sprite is litterally a bigger srpite of Great Deep Souls with actual Twin Humanities looking thing inside, the description is : "A sorcery that fires great dark soul dregs that have stewed for ages, far within the deep.

This sorcery is the highest form of Deep Soul

Some of the murkmen who rise from the depths are possessed by soul dregs, which have a grave likeness to the human form" This pretty much confirm all of this, humanity that has been corrupted by the Abyss started to form the Deep, however what if some corrupted humanity, with the help of the spell, had found a way to escaped, and since it belongs within human, found its way back into hollows, empty husk looking like Man... hiwever since it was corrupted, it mutates them, forming Pus-of-man

 I realize the Murkmen menrionned is the ennemy in the DLC however what if they found hollows instead?

You could further theorize that at the beginning  (Oolacile) the Humanity want as corruoted as itnis by the time of Dark souls 3 where it is called the Deep... Oolacile's habitants were obviously corrupted, the presence of Manus being somewhat of a catalyst for the abyss/deep allowing the corruoted humanity to actively twist the appearance of their host, could have made them permanently transformed... later on the descendants of Oolacile inhabitants mutated further with the Abyss corrupting Humanity into what is now known into the Deep.

But what if the soul couldn't find an host within the Murkmen? what if it wondered until it found an host? the hollow... Due to them not having being mutated in the first place, the appearance of the mutation would not be the same as those of the murkmen, in fact the lack of a predisposition to a mutation similar to that found in the murkmen, would prevent an imidiate transformation... however when threatened, that Humanity would have been trigfered by the "fight or flight" instinct that exist within Man... However due to The humanity's corruption, the flight option woukd be inexistant, and the "fight" part woukd trigger a sudden bursting transformation of the host forming the Pus-of-man.

Aditional proof/assumption

The Human Pine Resine' sprite looks similar to the human dreg's sprite, it's description says as follows: " Charcoal pine resin rotted with human body fluids.

Temporarily applies dark to right-hand weapon.

Normally used in the Undead Settlement for preservation and burials, but can mature into this state, becoming a valuable substance, used in a certain ceremony. Often seen for trade at exorbitant prices."

As some of you may be aware, used humanity in Dark Souls one, (the humanity counter next to your health) was sometimes called "soft" or "liquid" humanity, indicating  the so call " human body fluid" to be a mixture of human remains mixed with liquid humanity. In Dark Souls 1, humanity was the most expensive consumable, mirroring the description saying the price of the resine was exorbitant...

 The desciption of humanity was : "Rare tiny black sprite found on corpses. Use to gain 1 humanity and restore a large amount of HP. This black sprite is called humanity, but little is known about its true nature. If the soul is the source of all life, then what distinguishes the humanity we hold within ourselves?" This would seem to indicate that Souls and humanity are two different things, however it is pretty much confirmed that Humanity  came from the process if solitting the Original Dark Soul found by the Furtive Pigmy, in Dark Souls 3, you do not go hollow  (unless following Yuria Questline) however you can use Ember to achieve a similar process The Lord of Cinder, which boost your health as long as you don't die and loose it, similarly to Dark Souls 2's Human Effigy, the Ember's description says : " No Unkindled can ever truly claim the embers that burn within a champion’s bosom, which is precisely what makes their yearning for warmth so keen.

Gain the strength of flame and increase max HP until death.

With the strength of fire, the summoning signs of Unkindled become visible, and seekers of embers can be summoned to join in co-operation. But beware the embers may also attract invaders."

Similarly to Humanity, it would seems that embers are the result of Gwyn Splitting his own Lord Souls bewteen his Knights and the rest with the first flame when he rekindled it, from that you could assumed that Humanity and Ember are the same thing but from two types of Soul, One of Darkness, one of Light... It would also make sense that when you kill bosses, either Lord of Cinder themselves, or much like you, Heir of Cinder, you would gain Ember... Both of then are fragment of Lord Soul and only such souls can rekindled the first flame...