I'm a layman and the purpose of the board is to dive into uncharted territory so I'm not sure how useful a background in psychology-as-we-know-it would be anyway but here are a few wiki articles that are to the point of this thread.
Intro to what is currently understood.
http://en.wikipedia.org/wiki/Mass_psychogenic_illness
The section on modern outbreaks is informative because they were observed by modern medicine.
One good example:
Especially notable is the "June Bug" outbreak:[12] In June 1962, a peak month in factory production, sixty two workers at the Montana Mills dressmaking factory experienced symptoms including severe nausea and breaking out on the skin. Most outbreaks occurred during the first shift, where four fifths of the workers were female. Of 62 total outbreaks, 59 were women. Entomologists and others were called in to discover the pathogen, but none was found. Kerchoff coordinated the interview of affected and unaffected workers at the factory and summarizes his findings:
Strain – those affected were more likely to work overtime frequently and provide the majority of the family income. Many were married with children.
Affected persons tended to deny their difficulties. Kerchoff postulates that such were “less likely to cope successfully under conditions of strain.”
Results seemed consistent with a model of social contagion. Groups of affected persons tended to have strong social ties.
Think like a gardener; you need the right soil to grow the right plant. Like a virus a meme needs the proper environment. It seems a "receptive" population means a lot.
For our purposes this means we need to learn to identify and create that perfect environment.