I don’t know enough to comment on the Hygiene / Old Friends hypothesis, but anecdotally it seems like almost everyone I know now has an allergy or autoimmune disorder of some sort. After giving birth, my sister suddenly developed something similar to celiac disease (but not quite the same; I don’t think doctors ever figured out exactly what it is) to where she can’t eat grains and a few other types of food as well. I…
IHMO An old myth, I believe: Your immune system needs something to fight, otherwise, it starts finding problems that aren't.
If you want to reduce inflammation you should exercise regularly. This seems obvious but, to me, people will come up with conspiracy after conspiracy to explain this. But the basics - moving a bunch - work really well. The American Lifestyle is progressively more and more sedentary, and we're blaming our woes on sunscreen or something.