The party is exploring a crypt and has come upon a number of mummies.
The party paladin has
Lay on Hands. Alternatively, you can expend 5 hit points from your pool of healing to cure the target of one disease or neutralize one poison affecting it.
and also has
Divine Health. By 3rd level, the divine magic flowing through you makes you immune to disease.
The mummies have (emphasis mine)
Rotting Fist. If the target is a creature, it must succeed on a DC 12 Constitution saving throw or be cursed with mummy rot. The cursed target can't regain hit points, and its hit point maximum decreases by 10 (3d6) for every 24 hours that elapse. If the curse reduces the target's hit point maximum to 0, the target dies, and its body turns to dust. The curse lasts until removed by the remove curse spell or other magic.
Clearly, the mummy rot is a curse.
Is it also a disease, and subject to the paladin's immunity to, and magical ability to cure, disease?
There are several features in the game, such as those of the paladin, which interact with diseases. There is not, as far as I can tell, any definition of what a disease is, or any stipulation that a disease cannot be something else as well as being a disease (for example, a poison or a curse), with the exception of the petrified condition.
In the DMG section on diseases (pp. 256, 257) we read:
An adventurer emerges from an ancient tomb, unopened for centuries, and soon finds herself suffering from a wasting illness.
Well, that fits the initial cause of my question.
and
The rules help describe the effects of the disease and how it can be cured, but the specifics of how a disease works aren't bound by a common set of rules…What matters is the story you want to tell.
Is the implication here that whether or not a particular effect counts as a disease is entirely subject to determination by the DM, based on their narrative goals? Or is it is simple as an effect needs to explicitly name itself as a disease to count as one?
Related questions that do not answer my question:
What happened to disease in D&D 5e?
This question was asked after the release of the PHB but apparently before the DMG. I think my question, by adding the context from the DMG, can stand on its own, but if the community decides it is similar enough to the older question that mine is a duplicate, I would be happy to put a bounty on the older question for a more modern answer. In addition to the DMG, I suspect that someone who can draw on Tomb of Annihilation (which I don't own) can comment effectively on whether something can be both a disease and a curse, since the plot of that adventure appears to involve resolving the "death curse: a wasting disease".
What counts as a disease?
This question lists a number of real world genetic, metabolic, and parasitic conditions and asks whether they count as a disease within the game. While it shares with my question the desire for a more rigorous outline of what constitutes a disease within-game, the examples it asks about are irrelevant to my question.
Best Answer
The feature tells you if it is a disease.
This search on D&D Beyond shows numerous monsters with disease features. For example, the diseased giant rat’s bite says:
This is just one of many, many monsters with disease features. The linked search overloads DDB’s search capacity at 35 results.
Similarly, the spells harm and contagion tell you they inflict diseases:
We also see diseases in published adventures clearly stated to be diseases. For example, we see in Tomb of Annihilation a section on the diseases you can contract in the jungles of Chult:
It then describes the effects of several diseases. Similarly, the adventure Frozen Sick published in Explorer’s Guide to Wildemount is about a special disease, frigid woe:
There’s no rule or definition for disease by which we might discern if a particular effect is a disease, so it is quite natural to assume that a feature is a disease only if it states it is a disease, as all of these examples have done.
Disease curing magic works on it: maybe it’s a disease.
Things get a little bit more grey when a feature can be cured by disease curing magic, but it isn’t explicitly called a disease. For example, rot grubs:
Does this count as a disease? Maybe. Personally, I would rule that immunity to disease would not protect from this, because it doesn’t say it’s a disease, and your über-robust immune system isn’t going to do anything to protect you from larger parasites like flesh burrowing grubs. But it is perfectly reasonable for the DM to rule otherwise.
Disease curing magic generally should not break curses.
Lesser restoration, which cures diseases, is a 2nd level spell. However, remove curse is a 3rd level spell. Ergo, curse breaking magic is generally different and stronger than disease curing magic. The mummy rot curse is a curse, not a disease, and lesser restoration is not a “similar magic” to the remove curse spell - it’s weaker. So the paladin’s lay on hands feature should not cure mummy rot.