Disease as a spirit

I have been considering switching to Mythras and Classic Fantasy (a variant of RuneQuest) for some time, since I feel it is a better fit for my campaign setting than the idiosyncrasies of D&D. Among other things, Mythras is based on the premise that the fantasy world has fantasy physics as opposed to D&D operating on the premise that the fantasy world operates like modern science describes the real world with magic crudely tacked on. For example, Mythras treats disease as a spirit rather than a pathogen. More information below the break.

What is a spirit and how does it differ from a fey or elemental?

My cosmology is built holistically. That means that I take into account how everything fits together, where things come from and where they go, rather than cobbling together a random assortment of fantasy stereotypes and expecting them to play well together. Spiritually is a foundation of my setting, which I will explain in detail.

Spirits primer, elementals expanded

5e doesn't have a spirit type, despite spirits being a more or less universal concept. I have now expanded elementals to fulfill the same role. Some explanation, mostly quotations from and references to OGL sources, follows.

Distinguishing elementals, spirits and undead

13 True Ways and Eidolons introduced the new types Elemental and Spirit. Sometimes the distinction between these and other types may be ambiguous. I will try my best to sort that out here. Later I should write a primer on spirits and related things.

Spirits are the souls and personifications of natural forces, abstract concepts and emotions. They may overlap with spectral undead, in which case the GM could decide that spectral undead automatically qualify as both undead/spirit types.

Elementals are seething masses of base matter animated by demons, spirits, undead or similar entities. Constructs and elementals are similar because their bodies are composed of base matter and both have animating spirits bound into them. Some sourcebooks have included spontaneously animating constructs. The distinction is that the two are tied to opposing magical winds: constructs to artifice, elementals to nature.

Constructs and undead are pretty similar, being animated masses of something which isn't alive but might have been in the past. The difference between them is that they are tied to difference magical winds: constructs are tied to artifice, undead to death.

Not everything made of something other than flesh is an elemental, however. It is entirely possible for beasts and humanoids to have alien physiology based on fire or crystals or whatever. The reason why they aren't constructs or elementals is because they have a living metabolism: they eat and reproduce just like other living creatures.

On Spirits

So here goes my primer on spirits and their consequences for the game world and rules.

Spirits are anthropomorphic personifications of abstract and physical concepts.
They lack body-soul duality. If you try to steal their soul or they try to possess someone else, no soulless body is left behind. If a spirit is damaged to the point of death it is usually banished rather than destroyed and may reform in the future. Spirits are generally sexless (but may express a gender) and unable to reproduce (but some have offspring with mortals), with special exceptions.

Sources: Encyclopaedia Divine: Shamans, Relics & Rituals: Excalibur, Mythos: The Animae

Three primary distinctions

Primordial spirits: The primordials were the first spirits to come into existence from the primordial chaos, predating the distinction between spiritual and physical. They personify abstract fundamental forces: the Archons of Order, the Achamoth of Chaos, and the Aeons of Balance. (These names are taken from Gnosticism, and unlike other monsters they loosely emulate their namesake.) [Aberrations, Abominations, Asuras?]

Terrestrial spirits: Everything in the material planes, even the planes themselves, has physical and a spiritual aspect or soul. Humans and their ilk have animi; animals, forests, rivers, mountains, and light have terrestrials. The latter exist outside of but still tethered to their physical counterparts. Most of the time terrestrials slumber in the near ethereal unless their physical counterpart is disturbed.

Ephemeral spirits: The thoughts and emotions of living things leave residue in their surroundings. As this ambient emotional energy collects it produces ephemera (sing. ephemeron). Ephemera are so named because they lack physical counterparts to anchor them and must consume the emotions that spawned them.

Other spiritual states

Subcategories of spirits include aberrations, celestials, elementals, fey, fiends and manifestations.

• Aberrations are spirits that originate from primordial chaos, outer space, nightmare worlds and aborted planes. They include aeni qael, akata, denizens of Leng, mâlites, proteans and qlippothim. [Origins in other planes of existence (primordial chaos?), outer space, nightmare worlds (dreams of mad gods?), aborted planes (a possible future, the previous universe, an alternate timeline?), and mad wizard experiment? (That's silly!)]
• Celestials are spirits born of of law and light or created by the gods thereof to oversee the workings of the world. They include angels, couatls, kami, lammasu, pegasi, psychopomps, shedu, sphinxes, and unicorns.
• Elementals are spirits born of the elemental chaos, representative of one or more elements. An elemental is a dual entity consisting of a soul inhabiting a body of base matter. The animating soul could be of any kind: spirit, demon, undead, etc.They include atronachs, gargoyles, genies, jyoti, mephits, sceaduinar, verms, and weirds.
• Fey are "souls that have been given one last chance at existence" (see The Complete Guide to Fey p8). Rather than a class of related spirits, to be fey is a state of being not unlike—albeit inverse to—undeath. They include azatas, blink dogs, dullahans, hags, nuckelavees, nymphs, and satyrs.
• Fiends are spirits born of evil. They include the endless varieties of demons and devils.
• Manifestations (see Relics & Rituals: Excalibur) are spirits born of strong desires and emotions. They include animate dreams and the spectral dead.
• Unborn (see Book of Hallowed Might)