When I worked at a wilderness summer camp I had a kid in my group that was deathly allergic to peanuts, so weeks before we had to rid the camp of them. He was about 13, and said when he was little they did that scratch allergy test on his arm and he was in a coma for six months. He carried and Epipen around but told me not to stress. If someone ate a snickers then touched a door knob, and he touched it days later he…
Slight tangent: How do people with an allergy that severe find out they have an allergy? For example people who know even someone's breath after eating peanuts would threaten their lives. Accidental exposures are likely much worse than that, so how do you end up learning about this severity without dying in the first place? Just running a full allergy test after an unexplained exposure to traces of "something", or...…
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In terms of 'cause' - we don't know, one or two other children of my sons generation in our family have food allergies, and an older brother thought he was allergic to fish though never tested.
We noticed the reaction after he had a round of antibiotics after he had infected eczema, quickly followed by my wife needing antibiotics which we are sure got to him through her milk, as he'd groan after breast feeding - before that we had plenty of nuts, sesame, milk and eggs in the house, without noticing an issue for him. But that is just a pet theory.