Illusion, like hope, springs eternal. Without illusion, life would certainly be intolerable and even impossible; one might almost say that the art of the good life is in choosing one’s illusions wisely. The person who says that he is under no illusions contradicts himself.

No subject provokes illusion more powerfully than that of health. At first sight, the title of Daniel Davis’s book might lead the unwary to suppose that Davis is yet another crank peddling an easy way – chewing sunflower seeds, say, or wearing red flannel underwear – to immortality. But in fact he is professor of immunology at Manchester University and his book is a layman’s introduction to the current state of the science of immunology. It is not an easy read, not because Davis is wilfully obscure but because his subject matter is extremely complex. Explanations should be as clear as possible, but not clearer than possible.

Any reader completely unfamiliar with the workings of the immune system is likely to be dumbstruck with admiration for its intricacy. The system is innately able both to recognise millions of alien substances that might enter and harm our bodies and to neutralise them, either by engulfing cells or