The effects of stress on immunity has been studied and shows that it even affects the number and types of microbes that can survive in the human GI tract. Probiotics have a lot to do with digestion, synthesis of vitamins such as vitamin K, B5, B6 and biotin, stimulation of immunity, and regulation of immunity. Many scientists consider the stool in the gut an organ system because it has more metabolic activity than any organ system in the body. We have to be careful about what we do to this ecosystem because it can cause serious biochemical changes that can lead to the leaky gut syndrome, overgrowth of serious infections such as candida and C. diff that can be lethal.
There are many causes for disordered microbial balances such as NSAIDs, antibiotics, steroids, chemotherapy, infections, inflammation, diet and much more.