In this way, and even before any minute and systematic study of bacteria,--i.e., before the inauguration of bacteriology as a separate department of scientific study,--it was practically established that the agencies which produce putrefactive changes or fermentation were minute particles which were ever present in almost every substance, and that by heat or something corresponding to filtration it was possible to remove them or destroy their activity.
So much had been established without reference to the etiology of disease. In order now to study the germ-theory of disease as applied to man we must go back a little, neglecting the vagaries or the pure conjectures of the ancients, to the era of pure philosophic speculation,--perhaps to the days of Needham and Buffon.--to the middle of the previous century, when scientists and naturalists began to discuss the so-called spontaneous generation of life; for it is well known that fluids, like milk and others, abound with life after a few days of exposure; and it was supposed that the living organisms it contained had a spontaneous origin. This question of the spontaneous beginning of minute living forms was agitated for a century, or practically until Tyndall and Pasteur gave it its death-blow by their accurate and convincing demonstrations. There was no lark of experimentation, but there was lack of exact knowledge and of accurate deduction from facts observed. The bacteria--which at that time were usually spoken of as "monads" and "vibriones," because of their spontaneous motion--were found under varying circumstances, which, not being scientifically inquired into, led thinking men into a most perplexing condition{ 321} of mind. The two most ardent recent advocates of spontaneous generation were Bastian, of England, who wrote an elaborate treatise upon the subject, and Jeffries Wyman, of Cambridge, Mass., who gave it the benefit of all his influence. But, under the influence of blows dealt from the side of the physical laboratory by Tyndall, and from that of the biologists by Pasteur, the theory was weakened and effectually killed, so that to-day no one thinks of such a thing. On the contrary, life seems to be inevitably the gift of a preceding organism; and while the real origin of life is as unknown to-day as ever, there is not a single fact in the possession of scientists now justifying the view that life can have a spontaneous origin. Moreover the researches of Pasteur and others into alcoholic fermentation and the rôle played by the minute yeast-plant, and the early researches of Pasteur, Davaine, and Koch into the rôle of micro-organisms in producing disease in animals, and the scientific and elaborate study of bacteria and vegetable molds, inaugurated by Cohn and continued by many others, have as their legitimate outcome the creation of bacteriology as a science, and the establishment of the fact that the real condition in the so-called infectious diseases is one of fermentative or putrefactive alterations in the fluids and tissues of the living body, corresponding in minutiæ to the changes produced in saccharine fluids by the yeast-plant, or in decomposing animal or vegetable matter by the many known bacteria which are capable of producing such changes. To put it in another way, disease is simply an expression of the fact that these minute organisms, which are visible only under high powers of the microscope and which reproduce their kind with astonishing rapidity, gaining access to the surface or interior of the body, begin there to thrive and multiply, taking up from the living animal material for their own nourishment, thus robbing their host of that upon which his tissues must live, while at the same time, as the result of their activity, they produce{ 322} various substances which, so far as they are concerned, are excretory in nature, and many of which are extremely poisonous to the animal organism which harbors them. Such a disease as puerperal fever, for instance, is simply an expression of the fact that within the living human body there is going on active putrefactive action by which the internal cells are being destroyed. this destruction being progressive and often far-reaching; and that, as the result of their presence in the still living body, the noxious or toxic excretory materials of which they get rid are absorbed, in consequence of which such varying symptoms as nausea, fever, purging, vomiting, delirium, and many other symptoms are produced, the objective evidence of their local activity being the actual destruction of tissues, as is seen in cases of abscess, phlegmonous erysipelas, etc.