I have read it. I liked the book, but it was not what I expected. I do not think we even ever learned who the baby's mother was. I wanted to know - it is the mystery reader in me that wants it "solved". But, the crazy reaction of that town makes me feel ok that we never learned.
I think the convulsions were psychosomatic. So much stress/worry led to a physical reaction in Mercy, and it "spread" to her teammates. There could be a connection with the chemical plant to explain some of the physical symptoms, but that would be the secondary reason in my mind.