Oddly enough, the chances of being poisoned don't depend on the resistances at all. It's a function of endurance, if I recall correctly, and I also think that these low-level poisonous animals (cobras, spiders, rats) have multiple attempts to poison/disease your characters.(Yes, it's true. My magical monster-reference says that rat inflicts Disease I or II "×2". I was never sure about this, and nobody from the community ever was, but I think it means that it has two attempts to inflict disease. The probability increase would be considerable -- for example, if it had 40% chance to disease you, now it has 56%.)
(Some creatures have up to 5 attempts (I think it's Cobra with its poison) == nearly irresistible.)
