yes people can be intolerant to sth from one day to the other...
I'm the best example for it, because I got a histamine intolerance. Almost 3 years ago I got the diagnosis. I could'nt eat certain kind of fruits and vegetables as well. I always had this weird feeling in my mouth and I couldn't enjoy the meal anymore. Then I found a supplement called Daosin. Created by a company in austria which helps people with histamine intolerance. They are also working with some medical institution. For me it helps to enjoy my life :)
Absolutely, a persons allergies can change every 5 years, so, what you may not be allergic to now, you could be later, and vice-a-versa, something you are not allergic now, you could develop an allergy too. They are tricky and often hard to handle.
That's what's so scary is not knowing. For someone who already has a restricted diet because of allergies developing a food intolerance from the only foods you can eat is like a slap in the face.
that's not true :) I'm sensitive to histamine but with my supplement "Daosin" I can eat everything. So it's not a slap in my face ;P