Conversion is, according to Katherine Howe's description in the backmatter of this book, a mash-up of a case of conversion disorder that had recently hit the news, and her re-reading of The Crucible in a class she was teaching. She has woven together the story of the Salem witch trials with a modern outbreak of conversion disorder suffered by a number of high school students.
The stories aren't connected so much as they are strung along in tandem so that the reader can draw their own parallels and conclusions. And if it sounds like I'm giving away a little more of the plot lines than I usually do, its only because everyone knows the Salem witch trails, and I don't think anyone will be surprised to hear how that ends.
I guess this falls into the teen lit category. It was a fast read, and there were some fun parts, but I didn't love it. There was some insight into how high school girls behave, especially around each another, that rang true to me.