Florence Nightingale steadfastly refused to believe in bacteria, but she was wrong. The horrid truth is that every one of us carries billions of fellow-travellers, and no amount of bathing can ever change their number. The good news, however, is that most of our resident flora and fauna are harmless, or actually beneficial to our health. In 1976, Michael Andrews published these tidings in his bestselling The Life that Lives on Man, with all the details of our intimate companions in the micro-deserts of our forearms and the swamps of our underarms. But he failed to convince me that such slithy beasts as parasitic roundworms and liver flukes were equally benign. So I tried some internal experiments of my own.