Means of Invasion.—­Bacteriology has shown us that different species of parasitic bacteria have different means of entering the body, and that each must enter the proper place in order to get a foothold.  After we learn that typhoid infectious material must enter the mouth in order to produce the disease; that tuberculosis may find entrance through the nose in breathing, while types of blood poisoning enter only through wounds or broken skin, we learn at once fundamental facts as to the proper methods of meeting these dangers.  We learn that with some diseases care exercised to prevent the swallowing of infectious material is sufficient to prevent contagion, while with others this is entirely insufficient.  When all these facts are understood it is almost always perfectly possible to avoid contagion; and as these facts become more and more widely known direct contagion is sure to become less frequent.

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