Fatty was poisoned by the hemp red fruit when he ate the magic fruit brew. Because Fatty's poison did not need to be treated and it would clear up on its own, so everyone didn't care at that time.
But when Fatty took the divine fruit brew, it had a different effect from everyone else. In addition to bringing breakthroughs in Fatty's cultivation. Because there are toxins in Fatty's body, the divine fruit brew full of life would automatically transform into some detoxification particles. The ability to be invulnerable to all poisons.
Fatty had this ability because he discovered it once when everyone went deep into the primeval forest to cut down old trees in order to collect wood for building houses.
At that time, after being busy for a while, Fatty hid behind an old tree, being lazy. Unexpectedly, as soon as he lay down, he was bitten by a poisonous snake hidden under the leaves.