We're clearly at the very beginning of this. Using someone else's gut flora in the hope that it will sort out yours is on a par with pre-scientific herbal medic

We're clearly at the very beginning of this. Using someone else's gut flora in the hope that it will sort out yours is on a par with pre-scientific herbal medicine: trial and error, and it often worked, but you don't know why. In the future the medical system will analyse your gut flora and identify the specific mix of strains you need to take to remedy the situation.