Haha that's really funny!!! I posted the same post above!! I did not even see your post before I posted the same question at eh same time!!!
LMAO YET WE STILL DON'T HAVE AN ANSWER
I find it very unlikely he's immune. He knew about it and treated himself after eating. Bum was so reluctant to swallow there's no way he couldn't have known something was in the soup. He might have even planted the poison in the closet expecting Bum to get it. He also knew Bum needed to induce vomiting to get better. If Bum was suddenly overcome by an illness that was unknown to Sangwoo he probably would have treated it like the flu and would not have given him the salt water. The rat poison they ate might not have been too potent either. Bum is underweight so the effects of the poison would have came faster for him and a smaller amount would be fatal to Bum.
Oh damn thank you this makes much more sense rather than my theory of bum's parents being rats and that the poison could only affect him cuz he's part rat
THANK YOU VERY MUCH FAM
Someone please explain to me how sangwoo was not affected by the rat poison but bum was?? IS HE IMMUNE TO RAT POISON?!?