If placebo works by helping the brain decide that we should get better, causing it to release its own defensive chemicals then it stands to reason that other actions might also be able to induce the same response. The mechanisms involved pre-exist the entire concept of "drugs". Maybe the old-time "placebo" was going to the local healer who jumped up and down and proclaimed that you will get better.
Derren Brown's videos come to mind, especially his bit on the subway where he convincingly causes a man to become incapable of recalling where he is going, simply by commanding him to not remember.
If other people can cause such powerful changes in our brain function, then there should be ways to do it yourself.
I wouldn't put it past Derren Brown to have represented that situation on the subway somewhat differently than it actually happened - he is an entertainer after all (his program about faith healing and religious beliefs was amusing). Your point about influence still stands of course.
Derren Brown's videos come to mind, especially his bit on the subway where he convincingly causes a man to become incapable of recalling where he is going, simply by commanding him to not remember.
If other people can cause such powerful changes in our brain function, then there should be ways to do it yourself.