Germ Theoryof Disease

Germ Theory of Disease. Ever since the end of the 17th century the notion that putrefaction and some forms of disease were associated with the development (in the decomposing substance or within the living body) of certain low forms of life, has been entertained. The researches conducted by Leeuwenhoek into the minute forms of life, which he discovered and studied in water, the tartar of teeth, saliva, etc., undoubtedly played an important part in directing attention to this subject; and, although Leeuwenhoek himself did not formulate any theory of a contagium vivum (that is, that contagions which produce disease are living parasites), his observations directed attention to a new field of inquiry, and were quickly taken up by the theorists of succeeding generations. The microscope employed by Leeuwenhoek, though a great improvement on anything that existed before his time, was necessarily very imperfect; and, as improved instruments came into use, more and more exact knowledge was obtainable with respect to the extremely minute forms of life. In 1836 the discovery of an organism in fermenting yeast was first announced, and the theory was broached that the fermentative process was brought about by the growth and development within the fermenting fluids of this microscopic yeast plant. Evidence quickly accumulated in corroboration of this theory; and Pasteur, by a wonderful series of researches, established on a firm basis the notion that fermentation was a vital process. He studied what are now known as the lactic, acetic, and butyric fermentations, and showed that in each instance special characteristic germs were at work. What had now been shown to hold with respect to fermentation was quickly extended to diseased processes, and Pasteur wets able to demonstrate that the disease of silkworms known as pebrine was due to a microscopic fungus. In 1849 Pollender observed that the juice expressed from the enlarged spleens of animals dead of splenic fever or anthrax contained multitudes of small rodlets, which were, he maintained, vegetable organisms allied to what were then known as vibrio bacilli; and he suggested that these rodlets were the cause of the disease. Davaine developed this discovery, and proved a cause and effect relationship between the bacilli and the disease. Meantime the study of the group of microscopic fungi, called by Naegeli schizovtycetes or fission fungi, was being earnestly prosecuted, and the relationship between certain diseases and this group of minute organisms became more and more eipparent. The notion that the development of germs upon wounded surfaces led to suppuration and the breaking down of tissue, suggested to Lister the advisability of employing germicidal substances in surgical dressings, and the great revolution of treatment which is implied in the term "antiseptic surgery" became established. Pyaemia and septicemia were thus shown to be associated with bacterial growth; and, the germ theory being now well established, it became the fashion to discover a bacillus or micrococcus in every disease, and many hasty generalisations were made. More definite knowledge is now being gradually obtained, and the discovery in 1882 of the tubercle bacillus by Koch showed that one of the most common and destructive of morbid processes was caused by a germ; and not only this, the methods of study which Koch introduced gave promise of further achievements in this branch of research in the future. Koch further formulated four important requirements, which he maintained must be fulfilled before it could be considered that a cause and effect relationship between a given organism and a particular disease was demonstrated. (1) The germ must be shown to be present in the blood or tissues of the diseased animal; (2) this germ must be isolated from such blood or tissues, and obtained in "pure cultivation;" (3) this same pure cultivation, when inoculated into a healthy animal, must be cajiable of producing the disease in question; (4) the same germs must be again obtained from the blood or tissues of the diseased animal. It has not hitherto been found possible to fulfil all Koch's requirements in the case of many of the' suspected disease-producing organisms, but in the case of leprosy, relapsing fever, typhoid fever, malaria, diphtheria, erysipelas, and actinomycosis, the evidence is nearly as conclusive as in the completely established instances of anthrax and tubercle; while there is very good ground for supposing that it will be possible in the near future to make out a clear case in several other diseases. It may be remarked, in conclusion, that there is much reason for suspecting that certain low forms of animal life, quite distinct from the vegetable bacteria, are to be regarded equally with the fission fungi as disease-producing agents.