I suspect that if you haven't heard about the Le Roy girls - victims of mysterious undiagnosed twitching - you've been living under a rock. The story's been everywhere, but I like this article in the New York Times magazine. (great photography, too. This photo's by Gillian Laub, from the article:)
In short, a bunch of teenage girls in a has-been industrial town in New York came down with tics around the same time. One after another - first it was the "it" girls, the cheerleaders, the bubbly ones, but eventually it trickled down to the shy, the emo, the left out - the girls fell victim to uncontrolled spasming, jerking, and twitching so severe they hurt themselves and couldn't go to school.
What you expect happened. Old industrial accidents were blamed; Erin Brokovich rode into town at one point, loaded for bear. Blah blah blah. So far it's not really interesting enough to carry a book.
Until you get to the mass psychogenic illness theories. Even these aren't terribly original, but they're a starting point. Can you create an epidemic with the power of suggestion? What if your target is vulnerable in some way - teens, for instance (V-for-vulnerable is their middle initial, wedged between I-for-intractible and S-for-Stubborn, if my own are any kind of example)? How about a nursing home population? Fans of a losing team? Backers of unpopular legislation?
Good, good, but so far, even if you throw in some dead bodies, it's still just a middling Law-and-Order episode.
Read the NYT article, however (kudos to author Susan Dominus; her features are always captivating) and you'll discover a comprehensible and interesting discussion of something called a hysterical epidemic. And here, finally, is where I think the roots of a great thriller might lie.
Dominus quotes a book called HYSTORIES by feminist critic Elaine Showalter, who says a hysterical epidemic requires 3 ingredients:
1. Physician-enthusiasts and theorists
2. unhappy and vulnerable patients
3. supportive cultural environments.
Well, well, well. 1 & 2 are deep character work. 3 is setting. What's so special about that? These are where you can add depth and richness to a thriller plot, my friends!
The conflict comes in when you pit the enviro-crazies (oops, my bad - said with affection xo) against those who are invested in believing the situation is all in people's heads. Your challenge is to keep stacking the deck against each population, giving them more and more reasons to be dug-in with their beliefs. Their need for their brand of truth must be strong that it trumps all their other values - in other words, they need to be willing to kill to protect it.
And of course, you get points for making the epidemic interesting. Showalter offers help here - from the synopsis of her book:
Hysteria has traditionally been seen as a female disorder but in this study of its cultural implications, the author argues that it is a universal illness and that far from dying out with the end of the Victorian sexual repression it is becoming more widespread and manifest. Showalter identifies Gulf War syndrome, recovered memory, chronic fatigue syndrome, even claims of ritual satanic abuse, as the contemporary forms of the illness and by recognizing its universality releases women from the limiting association with hysteria.