Say you're a beekeeper. One of the hazards of your occupation is that you occasionally get stung. It doesn't bother you much. But one day, you're out doing your job, and you get hit by a couple of bees — no big deal, you've been tending bees for years and have been stung hundreds of times. You go about your business, but suddenly your lips start to puff up like croissants, your tongue feels like a rock in your mouth, your throat starts to close, you feel faint and maniacally itchy. You're rushed to the hospital, and it takes a megadose of epinephrine and an IV drip of who-knows-what to get you back to normal.
You're OK, but now you're apparently violently allergic to bees. The same might happen to adults who suddenly discover they can have a severe reaction to foods like shellfish, peanuts or tree nuts. What's going on?