I feel like a lot of it seems to come from living in more sterile environments which doesn't tone the immune system and then adding processed food and poison on top of it. One thing I noticed after starting practicing BJJ is that my immune system seemed to get alot stronger. I think it was a combination of regular exercise and contact with humans and all the germs that come with it. I remember when I took immunology in school the speculation at the time was allergies and autoimmunity issues may be related to low incidence of parasites from better sanitation and medicine. So because the immune system didn't have parasites to fight off it would start attacking itself. But very little concrete evidence to support the speculation.