We do not forget the plague, because it takes very good care of that, and is still, though we have long purses, the nightmare of health officials who keep their silent watch over the health of mankind. But, in dealing with epidemic diseases that have affected civilisation we must not forget malaria. Like syphilis itself, its social effects have been prodigious. Malaria was undoubtedly, with typhus and possibly smallpox, one of the real causes of the fall of the Roman Empire. The work of Sir Ronald Ross in discovering that it is spread by the anopheles mosquito, is one of the great epoch-making discoveries of history. There are few countries in the world which are free from malaria to-day. Unquestionably it is the cause of the anæmia and poor physique of the inhabitants of most tropical countries; and Australia is fortunate in having so few anopheles mosquitoes, though we have so many other varieties of the abominable little pests. That the numerous returned soldiers who were infected with malaria in Palestine have comparatively rarely spread their disease is due to the rarity of the anopheles in Australia; but doctors are always on the lookout for this particular pest and let us hope will keep it to its natural habitat in moister countries. Probably the real reason of the good physique of Australians is that the British race, for the first time in history, has had the chance of developing in a warm climate with no malaria. We are making a mistake in concentrating on the search for the discovery of the will-o’-the-wisp cause of cancer, which, though a very terrible and horrible disease, possibly associated into the very mystery of life itself, has probably less important social effects than either syphilis or malaria. I do not merely refer to the actual death-rate from these diseases, though it is probable that syphilis indirectly causes more deaths than cancer, tuberculosis or overeating. The trouble is that syphilis works in so mysterious a way; it is sly and subtle in its effects; it lies long latent and springs up again to the slaughter after the man who is its victim has long forgotten that he ever had it; and it conceals itself under innumerable guises. The great problem before the civilised world to-day is not the cure of cancer; it is the prevention of syphilis.