About 1 in 10 people have some sort of food allergy. It can be mild or it could be life threatening, but its VERY commonly mis-diagnosed because the symptoms seem to affect all the body's systems and occur differently in everyone. I have a wheat gluten problem http://forums.wrongdiagnosis.com/showthread.php?t=3857, and it was bad. The doctors never figured it out.
Now, as its being studied more, its being cited for a growing list of conditions. The list of symptoms is literally too long to list here.
Education can cure you! It might start in the stomach but it ends up affecting you everywhere.