Is there any way to overcome a paladin's immunity to disease without a gm fiat?
The goal is to make a paladin able to fall ill through any means possible without him losing his immunity, but instead somehow piercing it, be it with magic or through an item.
A poison that appears to be an illness.
Alternately, a Curse, like Mummy Rot. (Apparently this no longer works, since 3rd edition D&D started.)
Divine Health (Ex)
At 3rd level, a paladin is immune to all diseases, including supernatural and magical diseases, including mummy rot.
Functionally, to give a Paladin a normal any disease you will require GM fiat. Note that I am including researching a new magical gizmo under GM fiat.
You could consider ways to apply the sickened or nauseated conditions, though a Paladin with the Divine Health ability may easily be able to cure themselves of these conditions or at the very least suspend the effects for a few hours. If you found a way to permanently or continually give a Paladin these conditions, it may appear as though the Paladin has "fallen ill."
If you're looking into something a little more serious, your best bet would be to homebrew an exotic poison with a longer than usual frequency (one or more days).
Edit: Here's a Core Rulebook poison that might work:
King's sleep, ingested, DC 19, 1 day onset, frequency 1/day, 1 Con drain, 2 saves cures, 5,000 gp
It depends on trust and understanding between player and GM.
For example, if you were to have a campaign focused around a plague, it would be incredibly shortlived if a first-level cleric or paladin could simply cure it with their spells. Ideally, the GM should have discussed the possibility of plague as a campaign element in advance, and have explained that the regular cures, immunities and resistances will not work against this specific plague.
However...
The wording of your question, "without GM fiat", is the part I'm not sure about. The role of rules is different from group to group. For my group, the rules are a tool that the GM uses to resolve actions, and thus, all rules are GM "fiat". This again comes back to understanding between player and GM - provided the GM is fair and consistent, there should be no issue with afflicting the paladin with a disease, as the player should understand that it is being used as an important part of the game. In saying this, the GM should always have a good reason, and should never be arbitrarily taking away specific features that help to define a class.
