My go to example for this is http://en.wikipedia.org/wiki/Helminthic_therapy. That is the treatment of autoimmune diseases by infecting people with parasites which suppress your immune system in a targeted way for their own protection. I personally know someone who successfully treated his Crohn's disease with hookworm for 5 years and counting now, and the main problem he has is that if he ever needs antibiotics he needs to get reinfected because antibiotics will kill the hookworm.
But it is illegal to receive this treatment in the US because the FDA has never approved it.
I note it wasn't hookworm, which is what helped my friend.
The current pharmaceutical market unfortunately seems to behave in odd ways.
As a corporate entity, drug companies are capable of deploying investors' capital in a variety of ways. If they neglected the ways that actually made money, they would be short on investors pretty quickly.
Experiment [0] looks like a good example/PoC for this.
I'm a 6'5" 300 lbs male. I was taking upwards of 7g a day of vitamin c. Past that, I would get heavy flatulence, explosive diarrhea, and general gastronomic distress. I felt no change on my inside for doing excessive C, nor did I have any size effects other than the ones I noted for going past that.
One case, when I got sick, I was taking C. I wanted the illness to quit quickly, because I didn't want to be sick. So I took 8g C, 2 tablets of echinacea, and a NSAID. What happens was worse than any cold ever thought about being.
I killed off all my flora and fauna in my intestines. With this brought massive burning diarrhea (from the unneutralized stomach acid). I had this situation for 5 days straight. I was on the toilet at least 20x a day, day and night. Finally, day 6 had the formation of stool, which I cheered at. Day 7 and I was somewhat back to normal.