Above all, it is telling us what becomes of the pathogenic bacteria after being eliminated from the body of the patient; how they may exist for a long time still active; how they may lurk in filth or water dormant but alive, or how they may even multiply there. Preventive medicine is telling us how to destroy those thus lying in wait for a chance of infection, by discovering disinfectants and telling us especially where and when to use them. It has already taught us how to crush out certain forms of epidemics by the proper means of destroying bacteria, and is lessening the dangers from contagious diseases. In short, the study of bacteriology has brought us into a condition where we are no longer helpless in the presence of a raging epidemic. We no longer sit in helpless dismay, as did our ancestors, when an epidemic enters a community, but, knowing their causes and sources, set about at once to remove them. As a result, severe epidemics are becoming comparatively short-lived. BACTERIA IN SURGERY. In no line of preventive medicine has bacteriology been of so much value and so striking in its results as in surgery. Ever since surgery has been practised surgeons have had two difficulties to contend with. The first has been the shock resulting from the operation. This is dependent upon the extent of the operation, and must always be a part of a surgical operation. The second has been secondary effects following the operation. After the operation, even though it was successful, there were almost sure to arise secondary complications known as surgical fever, inflammation, blood poisoning, gangrene, etc., which frequently resulted fatally. These secondary complications were commonly much more serious than the shock of the operation, and it used to be the common occurrence for the patient to recover entirely from the shock, but yield to the fevers which followed. They appeared to be entirely unavoidable, and were indeed regarded as necessary parts of the healing of the wound. Too frequently it appeared that the greater the care taken with the patient the more likely he was to suffer from some of these troubles. The soldier who was treated on the battlefield and nursed in an improvised field hospital would frequently recover, while the soldier who had the fortune to be taken into the regular hospital, where greater care was possible, succumbed to hospital gangrene. All these facts were clearly recognised, but the surgeon, through ignorance of their cause, was helpless in the presence of these inflammatory troubles, and felt it always necessary to take them into consideration.