"But the symptom is real"

It is. I said so in chapter 1 and I'll say it again, because people with this habit are often told, or tell themselves, that it's all in their head. The calf is twitching. Anyone could see it.

A real symptom can still be an ordinary one. Anne McAteer and colleagues at the University of Aberdeen posted a questionnaire to adults registered with twenty general practices around the UK. It asked about twenty-five symptoms and whether people had had them in the previous two weeks. The average person had had between three and four. Some reported as many as twenty-two. The commonest were feeling tired or run down, headaches and joint pain. These were people living their normal lives, and most of those symptoms never reached a doctor.

So a healthy body has symptoms most of the time. It produces aches, flutters, twinges, rumbles, patches of numbness from sitting badly, and spots that come and go. Most are never explained, because they leave before anyone thinks to ask. If every one of them were searched, every one of them would appear on the symptom list of something dreadful, because those lists are built from the same ordinary signals.

Then there's what worry itself does. The NHS page on health anxiety says that anxiety can cause symptoms like headaches or a racing heartbeat, and that you may mistake these for signs of illness. Its page on twitching lists stress and anxiety first among the things twitches are often linked to, along with tiredness and caffeine. It adds that worrying can make a twitch worse. So the sequence on Tuesday night may have run like this. A stressful fortnight, poor sleep and three coffees produced a twitch. The search produced fear. The fear, and the bad night that followed, produced more twitching on Wednesday, which looked like the disease progressing.

Checking works in a similar way. When you stood on tiptoe at twenty to one, you were running a test that you've no training to interpret, on a leg you'd never tested before, while frightened and tired. Of course you couldn't tell whether it was weaker. Any result would have worried you, because you had nothing to compare it with. If you press a sore spot every day to see whether it still hurts, you can keep it sore. If you take your pulse when you're anxious, you'll find it fast. I think checking, like searching, is a search for certainty by a method that can't supply it, and it gives you more sensations to worry about.

A symptom can be real and harmless, and it can also be real and partly kept going by the worry and the checking. A search can't tell you which of those you have. Often time will, and if it doesn't, an examination will.