The planning and writing of this book has taken rather longer than I had originally intended; what began as a modest literary project for two second-year medical students has expanded over eight years to become a complete book. The subject matter lent itself all too easily to a sen- sationalist approach yet, on the other hand, a strictly scientific approach would probably have resulted in a dull dry text of little interest to the general reader. I have therefore attempted to bridge the gap and make the book intelligible and entertaining to the non-special- ist, but at the same time ensuring...