A study published in the recent Psychological Science journal explored the changes within our immunity which happen just from looking at people who are sick.  

The study took healthy volunteers and had them watch slides of people who were sneezing, etc. - people with basic cold/flu signs.  They found that by simply watching a slide of someone with this condition their own body would release an appropriate immune response as if they were in the presence of this person and needed the protection.

Very interesting study with many potential ramifications.  The most important of these is that while an immune response is good, having it happen too often is not necessarily the most desired outcome - particularly when the "threat" is only visual.  We would prefer our body use this vital energy only to protect us from "real" bacteria, viruses, etc.  

This makes you wonder what would be the effect of more serious conditions such as cancer.  Certainly there has been work done in the field of visualization therapy that has yielded some results with regards to tumor reduction, immune strength, etc.  These studies, however, have been far from consistent.  Perhaps doing visualizations in groups of other people with the same condition or even visually looking at the type of tumor (their own tumor perhaps) would lead to better results.