Diseases Are Undead

In the wonderful world of Dungeons & Dragons there are some things that are contagious.

You can catch being a ghoul. You can catch being a vampire. You can catch being a wight. You can catch being a wraith.

You cannot, to the best of my knowledge, catch any of these things in the real world which we all inhabit. You can, however, catch a cold.

I cannot quote you the precise page of rules or fluff that says you can also catch a cold in D&D, but it probably exists somewhere therein.

Which makes you wonder - right? You can catch ghoulery, you can catch the rhinovirus - maybe there's some overlap there.

In reality, where everyone who is reading or writing this definitely lives, contagious things are generally micro-organisms - bacteria, viruses, parasites, fungi, prions, etc. - which can hop from one body to another, through the air or by close contact and fomites or whatever else. In D&D, the generally-assumable world of D&D from monster manuals and setting gazetteers and suchlike that I'm familiar with, undead can spread because they're full of negative energy, which is the opposite of the positive energy which animates and invigorates living things, and when such an undead kills a living thing their negative energy can overflow into the now-energy-absent corpse to reanimate it as another undead. Something like that.

Besides undead, a D&D world, or at least D&D-ish world, also has gods and magic and other planes of existence and and dragons that can fly with wings that are quite small relative to their body mass so on and so forth, so I don't think we should or need to assume contagious disease works quite the same as in our world, our real world wherein we all currently exist. There could be fruitful gains from doing otherwise.

So I propose that contagious diseases are undead. Not all of these diseases are ghoul fever, or vampire flux, or whatever - you could simply re-fluff everything from the common cold to the black plague to your own invented nastiness as this undead phenomenon.

The ones you don't catch directly from getting scratched or bit or sucked off by a skeleton, those come from tombs. Graves. Catacombs. The dust of past civilizations.

Full of regret, loaded with resentment, pregnant with ghosts, every civilization bears a sickness - the negative energy of its collective animus. When a civilization falls, it loses its positive counter-balance to this animus. When a civilization falls this animus is fermented and concentrated in its ruins. Those who delve into these ruins without the proper protections become patient zero for the new-and-old disease within - new to the surface world, old with hatred beneath.

This is why the ruins are yet ripe with plunder for your party - only a small party could slip through the quarantine around them, and few sane rulers would risk their armies diving into a sickbed.

Buboes, polyps, sores, a bloodied cough, and so on? Footholds that the dust of ancient and unleashed undeath have on your body.

Some examples:

Tauthops, unleashed from the kurgans of the grassy steppe, cures the tendons of the infected like the bowstrings of the nomads who were buried within. Somewhat like tetanus, in that the affected are wracked by tension - and then the tension releases, launching the infected through the air - always in the same direction, until they dry, splinter, and burst. Every splinter carries the disease, and so pits are dug in the path of the advance of the infected, and soft leaves or blankets are piled atop them, to give them some comfort in their final hours and contain the splinters they release when they finally burst.

From the half-sunken sepulchers of the swampside port, profaned into curse-fended warehouses, there came the magpie sickness, a compulsion to swallow the shiny things one saw - and worse still when one swallowed enough for the stomach to split, for the blood and the bile and the sickness would mix to make a new maw from the fissured abdomen, which would spake unspeakable things.

Low on funds, the god-king cracked open the cliffside tombs of his divine ancestors, arrayed in the gold-gilt of their grave goods. He paid his mercenary-debt, but set loose the dog-gnaw, sores like those left by the fangs of a cur or baboon, which took the vital young before the toothless old, and left his holy kingdom bereft.