A fungal disease called rage rot (also known as “brain rot” and “skin rot”) is the result of spores released by a subterranean fungus infesting a host’s body. Some spores will find their way to just beneath the surface of the skin, where they begin to grow large, bulbous dry cysts; others embed themselves in the blood-brain barrier and begin to grow tumors.
The tumors cause excruciating headaches, as well as (in most species) producing bouts of uncontrollable rage (the headaches of course help these along): Hosts will unpredictably fly into violent rages and attack anyone nearby. In the ensuing struggle, some of the cysts on the host will burst, releasing clouds of spores that, especially given the victim is breathing hard in response to the fight, easily find their way into a new host.
Left untreated, the tumors will eventually kill the host. At this stage, the fungus consumes the corpse, while secreting a slimy substance that inhibits decay, prolonging its meal. It continues to produce the cysts, which periodically – especially if disturbed – burst and release more spores into the air.
There are treatments for it, which will halt the progression of the disease; while it doesn’t cure the tumors or the cysts, the latter can be (carefully!) removed afterwards, and no new ones will form. Those cured, however, continue to suffer the constant headaches the tumors cause for the rest of their life.
A few minutes in direct sunlight neutralizes the spores, so if a host dies on the surface – rather than in a dark tunnel somewhere – their corpse is relatively safe (from a short distance, anyway); still, to be sure, the recommended response to finding such a corpse is to immediately burn it, preferably from a distance and without otherwise disturbing it (i.e. don’t pick it up and carry it to a pyre, just burn it where it lies!).
Naja and Kobolds (but, interestingly, not Draconians nor Saurians) have a resistance to the disease, although they are not immune. Merrow are believed to be immune, although by virtue of not having lungs to breath in the spores in the first place; it’s unknown what would happen if the spores were to get into their gills, and it’s very unlikely we’ll ever know, as the spores float on the surface of the water and thus would not ever enter their gills. There are no known cases of it infecting Scylla, Arrigdrin, or Garuda, but it’s also not known if they’ve ever been exposed to it. Naiads are immune, but being that they are essentially “fallen angels” they’re immune to everything anyway. Dryads are also immune, as they don’t breath.