bacillus as obtained from the spleen of typhoid cadavers varies somewhat in different cases; that very similar bacilli may be obtained from waters contaminated by sewage, etc., which differ from the typhoid bacillus in certain characters of growth, and yet resemble it so closely that it is still uncertain whether they are to be considered distinct species or only varieties of the typhoid bacillus; that the character which was at first supposed to distinguish the typhoid bacillus from all others—viz., its invisible growth upon potato—has proved to be unreliable, inasmuch as certain other bacteria have been shown to have a like invisible growth, and under certain circumstances the typhoid bacillus may form a visible growth on potato. However, in spite of these difficulties in differentiating the typhoid bacillus from nearly allied bacteria found in water or in the dejecta of man and the lower animals, there is good reason to believe that the bacillus of Eberth, of Koch, and of Gaffky is the veritable etiological agent in the widely spread endemic and sometimes epidemic disease known as typhoid fever. But in admitting this we must admit that the bacillus itself is widely distributed, and that an attack of typhoid fever does not necessarily follow its introduction into the alimentary canal of man by means of contaminated water or milk. Other secondary causes, no doubt, often determine the question of infection. Among these we may mention individual susceptibility; exposure to agencies which reduce the vital resisting power, such as sewer-gas poisoning; the quantity and the virulence of the typhoid germs ingested; the state of the digestive function, etc.
In 1880 the present writer discovered the important pathogenic micrococcus which is now generally recognized as the usual cause of croupous pneumonia. This I now call Micrococcus pneumoniæ crouposæ: The German bacteriologists usually speak of it as the Diplococcus pneumonicB. I first discovered this micrococcus in the blood of rabbits inoculated with a few drops of my own saliva; and subsequent researches have shown that it is found in the saliva of healthy individuals in various parts of the world. This fact may at first view appear to be opposed to the statement that it is the usual cause of croupous pneumonia, especially as I have never myself suffered from this disease, although for several years I frequently demonstrated the presence of this micrococcus in my salivary secretions. But, as in the case of the typhoid bacillus and several other widely distributed bacteria, while accepting, upon experimental evidence, the etiological relation of the specific micro-organism, we are also obliged to admit the essential relation of predisposing or exciting causes in the development of an attack of the disease. In this connection I quote from a paper of my own, published in 1885: