If you get ill after eating or touching something that didn’t make anyone else sick, you may be allergic to it. Especially if there’s a rash. Allergies are caused by your body rejecting substances it doesn’t like. There is no treatment but to avoid those substances. Fortunately, only a few types of allergies can kill you. Nut allergies, for instance. Bee stings. But I imagine most people with fatal allergies to common things have died by now.
I am allergic to wool, soy, peanuts, and pollen. Only my peanut allergy can kill me.