I am intrigued by the relationship between the gut and the brain. This study that demonstrates outright behaviour changes is one example of our slowly growing understanding:
CBC News – Gut feeling: How intestinal bacteria may influence our moods
Even good old serotonin is now understood to be mostly produced by our ever-present friendly gut bacteria:
Microbes Help Produce Serotonin in Gut
http://www.caltech.edu/news/microbes-help-produce-serotonin-gut-46495 Medicine, nutrition and psychology are intimately intwined. Traditional means of understanding and teaching these disciplines will soon change forever.