OK, this is a simple version, and by all means not medical advice. Now that the disclaimer part is over.....
The stomach is lined with specialized cells called goblet cells. They are unique in that they can change their role as needed. They produce mucous to protect the stomach lining from erosion, and they produce acid for digestion. If more of one type or the other is needed, some will switch roles as needed.
Today's foods do some strange things to our digestion. If you eat only denatured foods (all cooked or canned foods, for instance) the acid tends to not mix in and do its job. (Now remember, I cook and can, so......I try to always have something raw or preferably something fermented with any cooked meal. Raw veggies draw in the digestive juices, cooked foods repel the digestive juices.) So your stomach will think there is a need for more acid since things aren't digesting, and will produce more. Hello, indigestion, acid reflux, GERD!
So you start to take Tums or other antacids....baking soda was what was used pre-Tums. These neutralize the acid.....so more of the goblet cells become acid producing, in an attempt to get the food digested.
Then you have GERD every day and the doc prescribes an acid-blocker, which shuts off the goblet cells. Eeek. So instead of that steak you just ate being worked on by acid, it sits in a neutral, watery stomach and doesn't break down much at all.
You likely now also have very few goblet cells that are working as mucous producers, so the lining of your stomach is left without the protection it needs. Ulcers.
Some ulcers can also be caused by certain types of bacteria, too.
But pretty soon, your digestive system starts shutting down in earnest, and you need meds to get it moving again. You start getting all kinds of symptoms that don't fit into any neat categories.....dysbiosis, or leaky gut syndrome. Your food is rotting and poisoning you. Think steak in 98.6 degrees for a day or two....or three....or four.
This is actually getting quite common......naturopaths tend to pick it up a lot more than MD's, sadly. Schools of naturopathic or homeopathic medicine are not influenced by big pharma, though. So the education is more wellness-oriented, usually.