We can, however, make our pedigree of typhoid a little more precise. It is not merely dirt of human origin which is injurious, but dirt of a particular type, namely, discharges from a previous case of the disease. Just as in the fight against malaria we have not the enormous problem of the extermination of all varieties of mosquito, but only of one particular genus, and only the infected specimens of that, so in typhoid, the contamination of water or food which we have to guard against is that from previous cases. From one point of view, this leaves the problem as wide as ever, for, obviously, the only way to insure against poisoning of water by typhoid discharges is to shut out absolutely all sewage contamination. On the other hand, it is of immense advantage in this regard,--it enables us to fight the enemy at both ends of the line, to turn his flank as well as crush his centre. While we are protecting our water-supplies against sewage, we can, in the meantime, render that sewage comparatively harmless by thoroughly disinfecting and sterilizing all discharges from every known case of the disease. A similar method is used in the fight against yellow fever and malaria. Not only are the breeding places of the two mosquito criminals broken up, but each known case of the disease is carefully screened, _so as to prevent the insects from becoming infected_, and thus able to transmit the disease to other human victims. It cannot be too emphatically insisted upon that every case of typhoid, like every case of yellow fever and of malaria, _comes from a previous case_. It is neither healthy nor exhilarating to drink a clear solution of sewage, no matter how dilute; but, as a matter of fact, it is astonishing how long communities may drink sewage-laden water with comparative impunity, so long as the sewage contains no typhoid discharges. One case of typhoid fever imported into a watershed will set a city in a blaze. The malevolent _Deus_ in the sewage _machina_ is, of course, a germ--the _Bacillus typhosus_ of Eberth. The astonishing recentness of much of our most important knowledge is nowhere better illustrated than in the case of typhoid. Although there had been vague descriptions of a fatal fever, slow and lingering in its character and accompanied by prolonged stupor and delirium, which was associated with camps and dirty cities and famines, from as far back as the age of Caesar, the first description clear enough to be recognizable was that of Willis, of an epidemic during the English civil war in 1643, both Royalist and Roundhead armies being seriously crippled by it. Since that time a smouldering, slowly spreading fever has been pretty constantly associated with armies in camps, besieged cities, filthy jails, and famines, to which accordingly have been given the names, familiar in historical literature, of "famine fever," "jail fever," and "military fever."