Could the microbes that inhabit our guts help explain that old idea of "gut feelings?" There's growing evidence that gut bacteria really might influence our minds.

Prions, long studied by UCSF professor and Nobel laureate Stanley Prusiner, build up in the brain to cause Alzheimer's and other dementias. Now they're linked to post-traumatic stress disorder in combat veterans and in the brain damage of athletes who have suffered repeated concussions.