Mind over matter in healing

I was reading Mark Twain’s small account on Christian Science, and he made a very good point about the power of the mind insofar as healing the body is concerned.

He said:

What’s up with this? If the mind is so important and powerful, how does it heal the body? Why don’t we use it more regularly? Why do we need medicine?

And importantly, are there any scienfitic studies that prove or disprove or explain this theory? How and why does it work, if indeed it does?



The above quote is from http://www.classicreader.com/read.php/sid.2/bookid.1286/sec.5/.