Recent investigations in the field of medicine turn over to the other sciences all questions regarding ultimate causes, and confine themselves to what is actually observed. We know that the agents responsible for infectious diseases are specific minute organisms which must be present in the system to produce the disease in question, and that these micro-organisms are conveyed from place to place by infected persons, by intermediaries, on articles to which they have attached themselves, in contaminated food, in drinking-water, and in many other ways. Investigators have studied the conditions in which these infective agents live and the manner in which they are disseminated, they have discovered methods of determining the nature of the disease in a very short time, and they have come to recognize the danger of coming in contact with germ-bearers, that is, with persons, healthy or convalescent, who have these micro-organisms in their systems without being themselves sick. Medical science is now endeavouring, by means of systematic procedure and splendid organization, to guard soldiers against the danger of infection; good drinking-water is provided, the men and the rooms in which they live are kept clean, persons suffering from infectious diseases are isolated, all rooms and articles used by patients are disinfected, infected divisions of troops are quartered by themselves, germ-bearers are watched for and discovered, &c. The success of such measures is well known. The knowledge gained and profited by in times of peace is also applied in times of war, and to-day we are able to confine pestilences within much narrower limits than was formerly possible. In order to do this, however, we must have, in addition to an efficient system of transporting and feeding troops, physicians who are well informed in regard to hygiene and bacteriology.