West Poison Au Yeung Fung's poisons were probably the most toxic in all of wuxia fiction. The most dramatic demonstration of his poisons' potency was Yeung Hong's death. Yeung Hong literally disintegrated from the inside out (and even the crows that fed on his remains died) within minutes after striking Wong Yung's soft body armor, which was still tainted after being stricken on that same spot by Gong Nam Freak # 4 Nam Hei Yan (who had previously been exposed to the poison). Even a Great such as North Beggar Hung 7 Gung was completely unable to access his inner power after exposure to the poison, a condition that lasted for two years and would have been permanent had Hung 7 Gung not gained access to the 9 Yum Jen Ging from Gwok Jing and Wong Yung. In DOMD, the potent acid that Wai Siu Bo used to dissolve the flesh and bone of corpses was also purportedly derived from Au Yeung Fung's poisons. This was clearly potent, potent stuff.
So that being said, is there anybody in Jin Yong's wulin who could have had complete immunity to all of Au Yeung Fung's poisons? How about Deun Yu with those poison-absorbing creatures that he swallowed (which protected him against pretty much all poisons he ran into in DGSD)? How about the Janitor Monk and Hui Juk, with their massive reserves of inner power far surpassing even that of a CONDOR HERO TRILOGY Great's? How about Au Yeung Fung himself? Or was Au Yeung Fung's poison so potent that nobody, not even Au Yeung Fung himself, would have been capable of immunizing himself against all of it?