I have a neighbor who's daughter is basically deadly allergic to just about everything anybody would consider normal. It's so bad that they struggle combining foods she can eat to get her complete nutrition...it's so bad that she's gone to the hospital with respiratory issues from accidentally coming in contact with somebody who had come in contact with one of her allergy triggers...
Her mother fought (and lost) a campaign to keep eggs out of school meals because they're one of the guaranteed-to-kill-her-daughter triggers. I had no idea how many foods have egg in them, but it appears to be just about all of them.
So you sit down and think about this, there is no way evolution could have possibly produced a person who's immune response to fairly normal environmental factors would be to send her into immediate shock, stop her breathing and kill her...it's just not possible, unless she's a random mutation, her entire genetic line would have been killed off the first time they encountered a wild grass or tried to eat a wild egg or whatever of the other couple dozen triggers she has. (I'm not ruling out that there may be some genetic predisposition to a wildly out of control immune system). So my conclusion is that it either has to be environmental (which genetics could make her predisposed to some combination of genes and environment that's gotten her here) or behavioral, the clean house theory (which I suppose also ends up being environmental in a way).
I think it's also possible, knowing her mother, that once she found out she was allergic to one thing, helicopter parenting stepped in and her exposure to other normative immune triggers was stopped, perhaps making things worse.
This year her mother finally started taking her to some kind exposure therapy where she's training her system to be able to handle eggs. Apparently so far successfully. So my guess is that she probably just needed to grow up getting dirty and eating all kinds of stuff to train her system.
Another story, my wife and I were out at he seashore some time ago for vacation. Of course we ate a bunch of seafood there, something my wife grew up with (growing up in South Korea) and I didn't (growing up a normal picky American). One night, her eyes and mouth swelled up and we determined she must have an allergy to something she ate (she has no other allergies we know of). When we got back we took her to an allergy specialist and they did a whole bunch of tests and she came back with some kind of alarmingly high measure in some blood test. They told her to stop eating some class of foods and a bunch of other stuff and to start carrying around an Epinephrine injector.
The thing is, that was it. They had no information at all for why she was suddenly "deathly" allergic to food she'd literally been eating weekly for her entire life (whatever the measure was in her blood, it was so high they sent it off to another senior specialist for further comments since they had never seen it that high in their office...an allergy specialist clinic).
They also had no information later on why, when she ate the same foods they warned her against (she's stubborn) she had absolutely zero reaction and continues to have no reaction to this day. The whole testing process looked reasonably scientific, with the grid of pin pricks and before and after blood draws and tables full of numbers from this or that blood factor. But in the end it seemed like the specialists knew no more about the subject than either of us, and we didn't know anything at all!
Very little has done more to shake my belief in modern medicine than the series of interactions with her doctors.