Medicine is moving more to the preventative lately in a trend that I find awesome! But some pretty cool studies have been done about people literally convincing themselves they have certain diseases and then getting the mock up version of them or years down the line, the actual thing. Cool thing about this is that it shows the power of positive thinking.
A guy in a controlled group was given the sugar pill instead of an actual med. he was depressed and tried to kill himself by overdosing on the pills he was given. He was rushed to the emergency room with dangerously low BP but when the docs contacted his group to see which pills they were, they found out he was the controlled group and when he realized this, all stats returned to norm.