There is no disease condition or damage type in D&D 5e as noted here in this Stack Exchange question. Hence no corresponding immunities, etc.
A case could be made that if a monster is immune to poison they are also immune to disease as two mechanics are similar when listed as an attack or effect. But in the RAW text no monsters and very few other things (land Druids, Monks, Paladins) are explicitly immune to disease.
It is my opinion that it is somewhat of an oversight as they are pretty consistent with other types of attack in what is immune and what is vulnerable.
As a final comment remember that rules in D&D 5e (or any RPG for that matter) are a means to help you, the referee, adjudicate what the players do as their characters in an imagined setting. The point of the game is the experience the players have as their characters in that setting. The point is not to play out the rules as written like it would be for chess, monopoly, or Settlers of Catan. So when the rules conflict with the logic of the setting (in D&D's case a fantasy setting) then the referee should use whatever the setting's logic dictates over the strict letter of the rules.
For most it doesn't make sense for a undead skeleton to be hit with a fever. So likely any ruling will be that skeletons are immune. In another example, it may make sense for a flesh golem to be effected by disease, but not for a stone golem.
Magic Circle and Protection from Good and Evil
If the Hag always finds the party about the same time of night, they could cast either in preparation for the hag to keep the Hag out. Protection would be better, as the Hag might forget to detect magic and waste the casting on an attempt to touch someone who is already protected.
Protection from Evil and Good is only an action to cast, so it can be cast easily even when the attack has started.
[W]hat does Magic Circle even do if you cast it on an area that already contains an excluded creature?
Traps the creature inside. The spell says:
When you cast this spell, you can elect to cause its magic to operate in the reverse direction, preventing a creature of the specified type from leaving the cylinder and protecting targets outside it.
So could trap the Hag -- but the problem here is that the casting Magic Circle takes 1 minute. Unless the DM allows you to hold completion of a spell similar to a ready action -- But that isn't RAW.
Oil of Etherealness
Applying the oil takes 10 minutes, and it lasts 1 hour. It will allow you to be on the ethereal plane. If you fight and break the touch the Hag, the Haunt attack is used for the day. If you secure the Heartstone, she can't get back to the ethereal plane, and if you kill the Hag, she'll be banished back to Hell.
Turn the Hag
Paladins of the Oath of Devotion's Turn the Unholy works when the Hags enters the material plane to attack.
Paladins of the Oath of Ancient's Turn the Faithless doesn't require Sight, if the Fey or Fiend hears the chanting they have to flee.
Paladins for the Oath of Vengeance can frighten a fey or fiend they see using Abjure Enemy.
Force Damage Won't Work, but Things of Force Do
A traveler on the Ethereal Plane is invisible and utterly silent to someone on the overlapped plane, and solid objects on the overlapped plane don't hamper the movement of a creature in the Border Ethereal The exceptions are certain magical effects (including anything made of magical force) and living beings.
However closer reading is that things made out of magical force hamper movement -- force damage doesn't pass through realms. This is made clear by a tweet from Jeremy Crawford:
https://twitter.com/JeremyECrawford/status/745062527118123009
No general rule causes force damage to pass from one plane of existence to another. #DnD
So while Magic Missle or Eldritch Blast won't help, the following will:
- Leomond's Tiny Hut
- Wall of Force
- Force Cage
Wake the Sleeper Will Not Work
Originally I thought that waking the sleeper would work. I said:
Simple answer are typically missed.
Always keep one party member on watch. If someone always keeps watch
(changing shifts so everyone gets full rest) they can look for party
members having nightmares. Having a nightmare of terrifying images
isn't a quiet thing -- ask any parent. Try putting the people with
highest perception on watch at the times you're most vulnerable, like
the middle of the night.
Once awake, the nightmare ends. If caught before an hour, the negative
effects don't take hold. The ability only works on sleeping people on
the material plane. The hags "Nightmare Haunting" is used up for for
that 24 hours.
However, RAI based on this tweet from Mike Mearls, that isn't the way it works. The victim only has to be asleep to start the nightmare haunt, but waking doesn't end it.
Alarm Won't Tell You She's coming
I thought Alarm might give you some warning, but its effect appears to be limited to the material plane. (Unless you cast it while in the Ethereal?)
Best Answer
They are immune to the spell
I will begin with the statement that all spell descriptions are rules, the text is not merely flavor text. In fact, the spellcasting rules refer to everything in the spell's description as the effect of the spell (emphasis mine):
The description for harm, as noted in the question, begins:
The effect of this spell is to unleash a disease upon the target. Both the damage and the reduced hit point maximum are the result of this virulent disease. If the target of harm is immune to all disease, via the paladin's Divine Health class feature or any other means, they would be immune to any effects of the disease. In this case that would confer immunity to both the damage caused by the disease as well as the lasting effect.
Therefore, it follows that disease immunity gives full immunity from the harm spell and any other spell that specifically causes its effect through disease.
This ruling is also supported (in an unofficial capacity) by a tweet from lead rules designer Jeremy Crawford: