Thanks to epistemocrat for tipping me off to this subject. Clostridium difficile is an infection that leads to chronic diarrhea and sometimes death. A controversial treatment is to take healthy bacteria (from a relative's feces), recondition it, and then transplant it into the infected person.
Studies show that over 90% of patients are cured with this type of treatment, often with just one treatment. Antibiotics don't work because they wipe out the good bacteria but not the C. diff bacteria.
This is another example of treating a health problem indirectly by altering the balance of bacteria.