More from Seth Roberts on fermentation and bacteria, specifically with regards to probiotics:
My take is that our immune systems need a steady stream of foreign pathogens (e.g., bacteria) and pieces of pathogens (e.g., bacterial cell walls) to stay “awake”;. When your immune system is working properly you fight off all sorts of bacteria and viruses without noticing. When your immune system isn’t working properly it overreacts (allergies) and takes too long to react (infectious diseases). Weston Price found twelve communities eating traditional diets whose health was excellent. Their diets varied tremendously but one thing they had in common was daily consumption of fermented foods, including cheese, kefir, sauerkraut, and fermented fish. This supports Amy’s story right down to the dosage. If you don’t eat fermented foods, you might use hookworms, which excrete a steady stream of foreign substances into the blood. (Thanks, Tom.) Hookworms definitely reduce allergy symptoms; I don’t think anyone has asked if they reduce colds and other infections.