Thinking Makes It So?

I guess there are some situations in which the title is true.

According to Flink, hypochondriacs are suffering from a form of ‘health anxiety’ - a clinical illness where the symptom is an obsessive interest in symptoms. Some of his patients are afraid of shaking hands or sitting on a toilet for fear of getting sick.

Er, as Brian said in Monty Python's Life of Brian "What chance does that give me?"