The writer has drawn as much as possible from original sources; this applies at least to the pestilences of the Napoleonic Period, and to the epidemic of small-pox after the Franco-German War. It would have been impossible to deal with the other wars in the same way without consuming considerable time. From the bibliographies it will appear what sources the author has consulted; rarely are quotations given from works which he has not seen, and in such cases it is indicated whence they were taken. The causes of the origin and spread of pestilences during a war are clear. Every aggregation of people, even in times of peace, at celebrations and annual fairs, in barracks, and so forth, is necessarily exposed to the danger of pestilence; but this danger is ten times as great in large assemblages of troops during a war. The soldiers are then subjected to all possible kinds of hardship and suffering—lack of food, or food which is inferior and badly cooked, sleeping out in the cold and rain, fatiguing marches, constant excitement, and homesickness—and all these things greatly lessen their power of resistance. When large bodies of troops are obliged to remain in one and the same place for a considerable length of time, the additional difficulty presents itself of keeping the locality unpolluted by the excrement of men and animals, and by refuse of all kinds. If an infectious disease reveals its presence in such an aggregation of people, energetic and stringent measures must be adopted, even in times of peace, to prevent it from spreading. In war times it is often impossible to take the necessary precautions, since the attention of the commanders is directed toward very definite objects, to which all other considerations are subordinate. Whether the germ of the disease is already in the place, or whether the soldiers bring it with them, in either case there is danger that the fighting armies will cause the disease to spread over the entire scene of the war, and thus seriously endanger thousands of human lives. Modern methods of sanitation have done much toward preventing the spread of army pestilences, not only in peace, but also in war. The last few decades have evinced that fact. Whatever attitude we may assume toward the question whether war can ever be wholly abolished, we must all agree that, if war has once broken out, all possible means must be employed to prevent the spreading of pestilence within the armies. Here the interests of the people and of the commanders coincide, since the efficiency of armies is often seriously interfered with by the outbreak of pestilence, and not infrequently the success or failure of a war depends, not upon the outcome of its battles, but upon the appearance or non-appearance of pestilence. CHAPTER I WAR PESTILENCES