I realised this four years ago, bacteria does indeed cause problems with the immune system and in some people even taking the wrong strain of probiotics can cause havoc on the immune system causing problems with fighting illness, tiredness, cold limbs, circulation problems, mitochondria dysfunction, Antibiotics can change your gut bacteria too leading to dysbiosis, seizures, low testosterone, rapid weight loss, hair loss etc... Your bacteria determines your overall health and how your immune system functions.