That's true. I remember hearing about a medical experiment in the 70's that dealt with children with leukemia.
Half of the kids were told to think about little-bitty "Luke Skywalkers" (remember this was in the 70's) waving around his light-saber inside their body, killing the cancerous cells with his weapon. It turned out that the kids who were told to visualize that scenario improved their condition dramatically. The control group did not do as well.
I don't have a citation for that experiment, it was something told to me (don't even remember who). But I may try to find it and read more about it.