Gluten is a protein found in many grains including rye, wheat, oats, spelt, barley, kamut and probably some other grains if we look for them. There are different levels of gluten "problems" including intolerance, true allergy, and viral-like reactions.

Intolerance - The body can digest some of the gluten but not very much, like it has an overload threshold.   This means it can digest some of the gluten but if you continue to consume it all the time then you develop local symptoms like bloating, gas, or feeling full all day, maybe because the body is missing an enzyme, a probiotic or some nutrient that aids in the digestion of gluten.  These symptoms usually go away after a few hours or days.

True Allergy - When you eat gluten you develop not only local digestive symptoms like above, but now you have a runny nose, cough, unable to exercise without shortness of breath, eventually you might develop joint pain, and start having systemic problems in which gluten will affect your heart, your joints - even causing arthritis, dry skin, depression and schizophrenia.  Also, gluten will cause severe intestinal and digestive problems which end up developing celiac disease, which means intestinal damage shown by biopsy.  Sometimes people on this level are diagnosed with irritable bowel syndrome, depression, schizophrenia, asthma, reflux, prostatitis, etc.  Physicians need a biopsy to truly diagnose celiac, although I don't believe in the biopsy because by the time you show damage to your intestines, too many things have already happened.  Other ways to diagnose celiac disease are Antiglidin antibodies, anti endomysial antibodies, tissue transglutaminase antibody, HLADq2, and DQ8 for genetical susceptibility.  The most accurate is doing the elimination diet for 14 days, but this has its weaknesses because if you are allergic to milk, and not gluten, but you have intolerance to gluten it might not show that you have intolerance because you are still having symptoms to the milk.

Viral Reaction - This is the worst one because the patient has seen a ton of doctors, had the biopsies, done the ineffective medicines, had every test in the book done but the symptoms are NOT in the gut, but in their immune system and heart.  If you eat one piece of gluten you’ll get so sick that doctors think you’re making it up or they’ll diagnose you with all kind of diseases every time they see you. 

The problem with gluten is that it is everywhere and in this society it’s been taught that they can't exist without bread, pasta, chips, soy sauce (yes, soy sauce has gluten in it), oats that are "gluten" free, etc. 

Once people are diagnosed with an allergy they decide to go to "gluten" free meals, and sometimes these are as bad as the original grain.

Gluten is a protein like what you find in bacteria walls, like some of the viruses that contain certain proteins, so the body treats it as such, and yes it is very prevalent.