The most recent theories of disease are the result of microscopical study of germs,--the germ-theory, in fact,--and{260} stand in the closest possible relation with the doctrine of spontaneous generation, fermentation, miasm, and contagion. In 1838 Ehrenberg regarded infusoria as animals, but Dujardin in 1841 expressed doubts, and Perty in 1852 affirmed that most forms classified as infusoria should be assigned to the vegetable kingdom, where, a little later, Naegeli relegated them. The correctness of this conclusion was proved by Cohn, who also perfected a classification. This particular form of investigation began in the twenties of the present century and assumed its present direction in the thirties and forties. Gaspard, in 1823, renewed the experiments of Haller, and injected into the veins of animals, not alone putrescent material, but the blood of other creatures suffering from the effect of such injections. Bassi, in 1835, discovered the cause of silk-worm disease, thereby giving special impulse to the theory of parasitism, and this was quickly followed by evidence of the existence of both vegetable and animal exciters of disease. Schoenlein, in 1839, demonstrated the fungus of favus; Vogel discovered the Oïdium albicans in 1840; Goodsir, the _Sarcina ventriculi_, in 1841; but the greatest influence upon the development of the parasitic, or germ-theory was the sequel to the discovery of the anthrax bacillus, by Davaine in 1850. In 1837 Latour and Schwann demonstrated that the cells, which were known even to Leeuwenhoeck, were actually vegetable forms, and Schulze had already pointed out that fermentation of fluids could only occur in the presence of extremely minute vegetable organisms; Chevreuil next showed that animal solids remained free from decomposition when protected from the access of germs; and in 1857 Pasteur demonstrated that fermentation and putrefaction were caused, not by chemical forms, as Liebig had taught, but simply by the agency of lower organisms, which he divided into aerobes and anaerobes; while in 1868 Chauveau queried as to whether morbific elements resided in the formed elements of germs or in their fluid{261} constituents. Thus the theory of _contagium vivum_, for which Henle contended as early as 1821, was not forgotten. In Germany Klebs and Hueter became the prominent champions of this theory; Hallier had designated his so-called "Microsporon septicum," and introduced a method of fractional cultures. The views of Klebs were opposed by Billroth, who contended for his "phlogistic ferment" and "_Coccobacteria septica_," upon which he wrote an elaborate and extensively illustrated treatise; he also at that time opposed the specific character of the lower organisms as disease agents. Hallier's _microsporon_ was refuted by Cohn, who studied and classified the various fungi, and distinguished between the pathogenic and the septicogenic,--that is, those which