A mad cardiologist can stop your heart, then start it again, make it beat in all kinds of strange rhythms, and install a pacemaker that can be used to regulate both the rate and rhythm. A mad dermatologist might be giving you topical cream to make that irritating rash worse, just to see how bad it can get. Some drugs that are used to treat cancer are so powerful that they are capable of causing cancer themselves, if given in dangerous doses. Cancers that are drug resistant or unsuitable for surgery can sometimes be treated with radiotherapy, where a tumour is given X-rays. But if it's the wrong dose or the wrong type of X-rays, there's always the danger of 'atomic mutation'. And, apart from the doctors themselves, what about the institutions in which they practice? Psychiatric institutions with security so efficient that once you get in it can be difficult to get out. Those huge old Victorian hospitals with their winding, poorly-lit corridors that may not lead where you're expecting. Now imagine being chased down them by someone, not at all sane, and carrying a very large needle.

Yes, it's not surprising that many of us harbour a fear of doctors. The thought of what a mad medic might do to you invokes the kind of very visceral, primal fear, that beats at the heart of the horror genre. Which probably explains why there are so many horror films about doctors, and not so many about mad bank managers or accountants. There are a few about lawyers, but I'm sure that will be dealt with in another book. Doctors are clever, doctors are skilled in doing things that can hurt us, and yet we must trust them when we are at our most vulnerable. To misquote Douglas Adams, you might think you're vulnerable in the shower, but that's just peanuts compared to when you have something seriously medically wrong with you. The idea of placing trust in someone whom you believe has greater knowledge than you has long been a fine source of potential terror and fearful situations, predating our age of science and reason. We trust teachers to look after our children in the daytime, handing them over to babysitters if we want to go out in the evening.

We trust those who prepare our food not to poison it, or to make it out of the most unappetising of ingredients. We trust our loved ones, our friends, ourselves. So many instances of trust, and all ripe for exploitation by skilled horror filmmakers.