The struggle of man against his unseen and silent enemies, the lower or bacterial forms of life, once one becomes alive to it, has an irresistible fascination. More dramatic than any novel, more sombre and terrifying than a battle fought in the dark, would be the intimate picture of the battle of our bodies against the hosts of disease. If we could see with the eye of the microscope and feel and hear with the delicacy of chemical and physical interactions between atoms, the heat and intensity and the savage relentlessness of that battle would blot out all perception of anything but itself. Just as there are sounds we cannot hear, and light we cannot see, so there is a world of small things, living in us and around us, which sways our destiny and carries astray the best laid schemes of our wills and personalities. The gradual development of an awareness, a realization of the power of this world of minute things, has been the index of progress in the bodily well-being of the human race through the centuries marking the rebirth of medicine after the sleep of the Dark Ages.