Do multiple cases of reduced HP from Harm need to be cured individually?

Question

If you fail your save against Harm, it will reduce your maximum HP for an hour or until it is cured via some kind of cure disease effect.

You unleash a virulent disease on a creature that you can see within range. ... If the target fails the saving throw, its hit point maximum is reduced for 1 hour by an amount equal to the necrotic damage it took. Any effect that removes a disease allows a creature's hit point maximum to return to normal before that time passes.

If a character has failed multiple saves against the harm spell and an ally applies some kind of disease curing effect on them (eg. lay on hands, greater restoration), will that cure all of the harm "diseases" at once or will each individual "disease" need to be cured?

Answer 1

You need to cure each application of Harm

First we'll start with effect stacking. The effects of multiple spells of the same name don't stack. However, as you've rightly pointed out, this doesn't matter.

The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect—such as the highest bonus—from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap.

So, the most potent application of Harm will be active. Any lesser applications of HP-reducing harm will be inactive, but present. There is nothing in the rules to suggest that spell effects are overwritten, but rather they are overshadowed. So if you reduce the hit points of a creature by 30 with one casting and 20 with another the next round, the creature's hit points will be reduced by 30 for one hour. One hour later, their hit points will be reduced by 20 for 1 round, since the 2nd casting would still be present on the target.

Why not all of them at once?

The ending of Harm reads thus:

Any effect that removes a disease allows a creature's hit point maximum to return to normal before that time passes.

There are a few ways of reading this. The first is a global application of this rule and the second is a contextual application. In the first, you could argue that all health-reducing effects would be removed along with the disease. This is the wrong way of reading it. To suggest that a single disease cured removes all applications of Harm would necessitate a global application of this rule. And that is just not feasible. This spell would be wildly out of scope. It would mean that you could remove a Vampire's health reducing effects via remove disease if the target were also afflicted with Harm. Spells as a rule only describe their own effects, not global effects for other spells unless they explicitly say they do.

The second way, therefore, requires that we treat this final clause as effecting only each individual casting of Harm. In the same way that each spell gets its own damage roll and saving throw, each effect must be considered it's own individual "disease". So one cure for one application of harm.

Answer 2

As a DM, I would rule that each instance of Harm counts a single disease (probably)

When the Harm spell says that it's removed by effects that remove diseases, it probably means to say that it is a disease with respect to those effects. The first sentence of Harm's description supports this interpretation:

You unleash a virulent disease on a creature that you can see within range.

This means that if someone is afflicted by multiple instances of Harm, they would have to be either removed one at a time by an effect like as Lay on Hands or either version of Restoration, or all at once by an effect that removes all diseases like Heal.

But if you read the rules literally, maybe not

Harm's description says the following:

Any effect that removes a disease allows a creature’s hit point maximum to return to normal

Read strictly, this means that a target who is affected by 1x Filth Fever and 10x Harm, the Harms would all end if the target Lesser Restoration cast on them to remove the Filth Fever, since they the Lesser Restoration is an effect that removes a disease, and therefore triggers that clause of the spell's effect.

Similarly, if the Lesser Restoration removed the disease from the Harm spell, that one would be removed directly while the other nine would have their removal clause triggered (leaving only the Filth Fever in place)

I strongly doubt that this is the design intent, however, since it is a counterintuitive reading of the rules, which likely would have been expressed more clearly if it were actually what they meant.