How do I determine what in-game features and effects constitute diseases?

Question

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.

Answer 1

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:

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 4 (1d4 + 2) piercing damage. If the target is a creature, it must succeed on a DC 10 Constitution saving throw or contract a disease. Until the disease is cured, the target can’t regain hit points except by magical means, and the target’s hit point maximum decreases by 3 (1d6) every 24 hours. If the target’s hit point maximum drops to 0 as a result of this disease, the target dies.

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:

You unleash a virulent disease on a creature that you can see within range. […] Any effect that removes a disease allows a creature's hit point maximum to return to normal before that time passes.

Your touch inflicts disease.

[…]

Since this spell induces a natural disease in its target, any effect that removes a disease or otherwise ameliorates a disease’s effects apply to it.

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:

The following diseases can affect giants and humanoids exploring the jungles of Chult. Remember that lesser restoration and similar magic can cure a disease.

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:

Frigid Woe. Frigid woe is a special disease developed by Aeor's mages that cannot be cured by conventional treatment or magic. The only way a creature infected with the disease can be cured is by finding and drinking the manufactured antidote, a milky liquid stored in gold vials found in Eiselcross's ruins. This disease was created to slow down the forces of the gods and get around the healing power of their clerics and angels.

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:

Bites. Melee Weapon Attack: +0 to hit, reach 0 ft., one creature in the swarm’s space. Hit: The target is infested by 1d4 rot grubs. At the start of each of the target’s turns, the target takes 1d6 piercing damage per rot grub infesting it. Applying fire to the bite wound before the end of the target’s next turn deals 1 fire damage to the target and kills these rot grubs. After this time, these rot grubs are too far under the skin to be burned.

If a target infested by rot grubs ends its turn with 0 hit points, it dies as the rot grubs burrow into its heart and kill it. Any effect that cures disease kills all rot grubs infesting the target.

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.

Answer 2

There is no hard rule

As you say, the main reason to care about how disease is defined is because game features like Lay on Hands can cure them, and players and DMs want to know what can and cannot be cured.

I think in practical terms, these features can cure

• anything that the game explicitly says is a disease
• anything the feature explicitly says can be cured
• anything the DM determines to be a disease

Unfortunately, there is no explicit or rigorous definition of disease in the game, so the corner cases will be a DM call.

In the case of mummy rot, it says it is a curse, it does not say it is a disease. It also says it can be cured by remove curse, a third level spell. Other magic that can break curses could also cure it.

Lesser Restoration could only cure it, if the DM decided to also treat it as a disease. Doing so would make mummy rot easier to overcome, as Lesser Restoration is only a second level spell (and because other features that can heal disease then also work).

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Game Definiton of disease

As cited in this answer to "What Counts as a Disease?", the closest thing we get to a definition of disease in game is given on p. 256 DMG, under the heading "Diseases":

A disease that does more than infect a few party members is primarily a plot device. 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. Diseases can affect any creature, and a given illness might or might not pass from one race or kind of creature to another.

While the term "infect" showcases that infectious diseases are certainly diseases, the rules make it clear that there are no common rules for diseases. Diseases may be contagious, or they may not be. So the concise, sharp defintion you are looking for does not exist.

Examples given

The section provides further examples:

A plague ravages the kingdom, setting the adventurers on a quest to find a cure. An adventurer emerges from an ancient tomb, unopened for centuries, and soon finds herself suffering from a wasting illness. A warlock offends some dark power and contracts a strange affliction that spreads whenever he casts spells.

A plague is caused by infections, and that seems to be the primariy kind of disease the game is concerned about, as the entire next paragraph is about outbreaks. But a wasting illness can have other causes, such as hereditary disorders that cause muscle atrophy, or chronic wasting disease, that appears to be caused by prions (malformed proteins). And the strange affliction that manifests when casting spells has nothing to do with natural, viral or bacterial infections either. So diseases can encompass more than just infections.

Dictionary Definition

By default we turn to real-world usage if the game fails to define a term. Unfortunately, that does not help so much here either, as even in the real world, what counts as a disease is not sharply defined and is complex.

For example, are infestations like tape worms diseases? Not by themselves, but they can lead to disease. How about extenral parasites, like lice? They generally are not, but there are epidermal parasitic skin diseases, so sometimes they are.

Dictionary defintions also do not help much. Here is the one from Oxford Dictionary

A disorder of structure or function in a human, animal, or plant, especially one that has a known cause and a distinctive group of symptoms, signs, or anatomical changes.

And here from Encylopedia Britannica:

disease, any harmful deviation from the normal structural or functional state of an organism, generally associated with certain signs and symptoms and differing in nature from physical injury. A diseased organism commonly exhibits signs or symptoms indicative of its abnormal state. Thus, the normal condition of an organism must be understood in order to recognize the hallmarks of disease. Nevertheless, a sharp demarcation between disease and health is not always apparent.

Unfortunately, these definitions would include all kinds of harmful magical alterations that can exist in the game world and have nothing to do with disease there (the people writing them did not have to think about exluding magic as a possible source of harmful alterations).

History of Mummy Rot

If Mummmy Rot were treated as a disease, it would be much easier to remove, with a simple level 2 Lesser Restoration instead of the Level 3 Remove Curse.

In general, for power balance reasons, if the game provides a specific spell that allows to overcome something, the principle is that other spells should only be able to also overcome it, if they are of equal or higher level.

For this question, it might be useful to look beyond 5th edition to understand how we should treat mummy rot.

In D&D 3.5:

Mummy Rot (Su)
Supernatural disease (...)
Unlike normal diseases, mummy rot continues until the victim reaches Constitution 0 (and dies) or is cured as described below.
Mummy rot is a powerful curse, not a natural disease. A character attempting to cast any conjuration (healing) spell on a creature afflicted with mummy rot must succeed on a DC 20 caster level check, or the spell has no effect on the afflicted character.

In AD&D 2e:

Mummies are horrific enemies. A single blow from one's arm inflicts 1-12 points of damage, and worse, its scabrous touch infects the victim with a rotting disease which is fatal in 1-6 months. For each month the rot progresses, the victim permanently loses 2 points of Charisma. The disease can be cured only with a cure disease spell. Cure wounds spells have no effect on a person inflicted with mummy rot and his wounds heal at 10% of the normal rate. A regenerate spell will restore damage but will not otherwise affect the course of the disease.

Cure Disease in 2e was a third-level spell like Remove Curse, not a second level spell.

In Ad&D 1e:

The scabrous touch of a mummy inflicts a rotting disease on ony hit. The disease will be fatal in 1-6 months, and each month it progresses the diseased creature loses 2 points of charisma, permanently. It can be cured only by a magic spell, cure disease.

Again Cure Disease in 1e was a third-level spell like Remove Curse.

From a historical perspective, Mummy Rot was a disease, but an especially tough one. It was much harder than in 5e to overcome diseases, and an encounter with a mummy was a frightening proposition for a low level party. I recall one adventure, where one of the PCs was hit by it, and that turned into a whole side quest racing against time to obtain a cure before the character died.