There a lots of unknowns, but it could be that humans have evolved around the micro-organisms just as much as they evolved around us, and they influence us in ways never imagined before. We could be the willing zombies for the 100 trillion bacteria inside each of us.
Mice given some parasites try to get eaten by cats. The same parasite in humans has been linked to schizophrenia and depression.
The cure — getting someone else's feces transplanted into your intestines.