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.