obsessed

What was starting to unsettle him, to frighten him, was the idea that Merry was less horrified now than curious, and soon he himself became obsessed, though not, like her, by the self-immolators in Vietnam but by the change of demeanor of his eleven-year-old.

Believing that an evil spirit is trying to obsess one is a dangerous belief, and when one comes to believe he is obsessed by an evil spirit, though there is not an evil spirit within a thousand miles of him, he will have all the symptoms.