Malaria has probably killed more human beings than all the wars that have ever devastated the globe. Some day the epic of medicine will be written, and will show what a large and unexpected part it has played in the progress of civilization. Valuable and essential to that progress as were the classic great discoveries of fire, ships, wheeled carriages, steam, gunpowder, and electricity, they are almost paralleled by the victories of sanitary science and medicine in the cure and prevention of that greatest disrupter of the social organism--disease. No sooner does the primitive human hive reach that degree of density which is the one indispensable condition of civilization, than it is apt to breed a pestilence which will decimate and even scatter it. Smallpox, cholera, and bubonic plague have blazed up at intervals in the centres of greatest congestion, to scourge and shatter the civilization that has bred them. No civilization could long make headway while it incurred the dangers from its own dirtiness; and to-day the most massive and imposing remains of past and gone empires are their aqueducts, their sewers, and their public baths. What chance has a community of building up a steady and efficient working force, or even an army large enough for adequate defense, when it has a constant death-rate of ten per cent per annum, and an ever recurrent one of twenty to thirty per cent, by the sweep of some pestilence? The bubonic plague alone is estimated to have slain thirty millions of people within two centuries in Mediaeval Europe, and to have turned whole provinces into little better than deserts.