Gourmet_DAO the MC has an allergy. He ate something that triggered his allergy. He convulsed for a day. According to past results of the same situation, he was supposed to be dead. However his body preserved. Every white cell of his body was defending the substance that attacked his body. Finally his white cells won. He survived. His white cell evolved because of this. It repairs itself 100M/ per second making him immune system became stronger than anyone else. Like wolverine, his wound healed instantly. He was then called, "Mr. Undead" (lame title I know. Can change that)
However, this unkillable man has weakness that no one knows.
He still has his allergy.