About 10 yrs ago I got very sick while overseas and was hospitalised. On getting back home I went to my local GP and when I said I was feeling mentally "out of it" and had gut problems he immediately diagnosed me as depressed and anxious and gave me medication!! I actually started to take it and I felt much much worse. Luckily at the same time I started researching on the net.
After much digging I found a paper linking gut biom with depression or anxiety. Basically infection with water borne bacteria can give the symptoms of depression as they "eat" seratonin which has been created in the gut. I went to a health shop and ask for a herbal bacterial cleanse. I took aweful tasting herbs for a month and the results were massive. Never have had a problem since.
My daughter who is a Nurse Practitioner mentioned this to a specialist at the hospital she was at at this time. The specialist said "No your father is incorrect". The next day this specialist came back down to my daughter's section and apologised and said "Yes I researched and he is correct".
Of course this is now becoming common knowledge.