One reason I believe we are vastly bacteria-deprived (and thus greatly benefit from fermented foods) is the efficacy of hookworm therapy: Hookworm parasites can reduce autoimmune diseases. Hookworms, like fermented foods, stimulate the immune system in a chronic, harmless, low-level way. Here is a good introduction to the subject:
Musician Scott Richards and artist Debora Wade are two Bay Area patients on the hookworm treatment. Richards and Wade both suffer from an inflammatory bowel disease called Crohn’s. When faced with using a parasite as therapy, both patients felt they had nothing to lose. . . . Both Richards and Wade say they didn’t have to wait long to feel relief. Richards [described] waking up and the pain suddenly gone. For Wade, she needed to be reinfected, but today said she can eat foods that patients with Crohn’s could never eat: pizza & Thai food for example.