IN THE organised chaos of Mary Sheppard's office she sees a small parcel has arrived on her desk. She doesn't need to open it to know what's inside - a human heart soaking in the preserving fluid formalin, and possibly some slides containing tissue samples neatly folded in bubble wrap. A specialist in heart and lung pathology at the Royal Brompton Hospital in London, Sheppard is the person to whom general pathologists round the country turn when they have no answers. This heart has been sent to her because nobody else can work out why it stopped beating.

Its owner, a 22-year-old man, died suddenly while swimming. The local pathologist determined that the man did not drown, yet could find no other cause of death. The man had no history of disease or illness, was fit and a strong swimmer, yet for no apparent reason had just died.

Such deaths ...