Man, its been AGES but dont you mean 3rd edition? This is the 2nd edition ruleset, I dont think paladins were immune to desease back then. Just wait for other people confirm or deny what I and Cheesebelly said.
Pretty sure my Paladin was 3rd level when we fought that ghast. I was using Ankhegs to level up quickly and I could of sworn he was at least 3rd level or 4th.
It was possible to get this immunity with the aTweaks mod. So if you want it, please wait for this mod to be available for BG:EE (no release date right now).
Possibly an oversight but I don't recall it has ever been a feature of the Infinity Engine games and certainly not BG1/BG2. I'm sure you could find mods for it but the devs made a lot of decisions on what rules they kept/lost/changed so BG doesn't exactly mirror 2nd Edition rules.
From the 2nd Edition Paladin's Handbook (C) 1994 TSR:
Immunity to Disease A paladin has complete immunity to all forms of organic disease. This immunity includes diseases from rat, otyugh, and neo-otyugh bites, as well as nonlethal but disabling maladies like measles and earaches. A paladin never catches a cold or suffers from tooth decay, and is unaffected by parasitic monsters such as green slime, violet fungi, gas spores, phycomids, and rot grubs. His wounds never become infected. The process occurs instantly, automatically, and painlessly; the paladin isn’t even aware of exposure to a disease. A paladin’s disease resistance is divinely given and can’t be transferred to another character. A magical blood transfusion, for instance, would not grant disease immunity to the recipient. Nor are a paladin’s progeny automatically immune to diseases, unless they become paladins themselves.
Psychological disorders. A paladin is as prone as anyone else to hallucinations, nightmares, memory loss, insanity, and similar mental and emotional disorders, providing the disorder isn’t a consequence of a disease (such as a fever-induced delirium).
(Some kits are not immune to disease).
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From PlanetBaldursGate.com:
Skills from AD&D that are not implimented[sic] in BG: resistance to disease (there is no disease in the game), Cure Disease, possession of a faithful war horse, and the necessity of giving away wealth and most possessions. In ToB, the Paladin can draw from a high level ability pool which gives the Paladin some extra abilities.
Remember: BG1 was always a modified version of 2nd edition. Do not compare it to the rulebook. Compare it to the original BG1. They made changes for balance and playability.
Comments
The Cavalier is immune to poison, but disease is a complete different category.
http://www.enworld.org/forum/d-d-pathfinder/257777-paladins-kyuss-worms-2.html
Immunity to Disease
A paladin has complete immunity to all forms of organic disease. This immunity includes diseases from rat, otyugh, and neo-otyugh bites, as well as nonlethal but disabling maladies like
measles and earaches. A paladin never catches a cold or suffers from tooth decay, and is unaffected
by parasitic monsters such as green slime, violet fungi, gas spores, phycomids, and rot grubs. His
wounds never become infected. The process occurs instantly, automatically, and painlessly; the
paladin isn’t even aware of exposure to a disease.
A paladin’s disease resistance is divinely given and can’t be transferred to another character. A
magical blood transfusion, for instance, would not grant disease immunity to the recipient. Nor are a
paladin’s progeny automatically immune to diseases, unless they become paladins themselves.
Psychological disorders. A paladin is as prone as anyone else to hallucinations, nightmares,
memory loss, insanity, and similar mental and emotional disorders, providing the disorder isn’t a
consequence of a disease (such as a fever-induced delirium).
(Some kits are not immune to disease).
-----------------------
From PlanetBaldursGate.com:
Skills from AD&D that are not implimented[sic] in BG: resistance to disease (there is no disease in the game), Cure Disease, possession of a faithful war horse, and the necessity of giving away wealth and most possessions. In ToB, the Paladin can draw from a high level ability pool which gives the Paladin some extra abilities.
No disease in game? Why is there a cure disease spell and how are Ghasts able to disease you.