In fifth edition D&D, do any of the manuals specify that a creature (besides players) can be immune to all diseases or is this left up to DM discretion?
Say for example that the party comes up against an iron golem and one party member wants to cast Contagion on the golem. Is there something somewhere that states that all nonliving things or constructs or creatures in some other category are immune to all disease or is that up to my judgment as a DM so that I can rule that, for example, unicorns are immune to all disease but iron golems can suffer from a disease that I made up called Rust Mange?
What I'm looking for is at least one instance of blanket disease immunity for a non player creature in the PHB, DMG, or MM or an indication that none can be found.
Best Answer
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.