16. Leaf affected by Hymenomycetous 11-15. Rusts. showing effects on leaves. 7-10. Potato Rot. 1-6. Brand-fungi (Ustilagineae) of Oats and Wheat, with Spores. Fungi (Oidium). 17. Oidium-Growth. 18-23. Ergot-fungus, showing development and effects. DISEASES of much useful information, but they are naturally subject to much change, as new discoveries ally certain heretofore apparently unrelated diseases under common heads. Consult Hektoen, 'American Text-book of Pathology.' Diseases, Germ Theory of. As a speculation, the germ theory of disease is centuries old; as demonstrable in fact, however, it is a thing of but yesterday. Aristotle in the second century believed that certain diseases were due to microscopical organisms, although he and many others who believed the same could not prove it, but it was reserved for Pasteur, Koch, and other modern students of the young science of bacteriology to demonstrate beyond doubt that certain disease processes result from the invasion of the human body, as well as those of animals and plants, by certain forms of lowly organized plant and animal life, and that the disturbance in the healthy functions of these animals and plants is an indication of the battle taking place between the living animal or plant cells and their animal or plant parasites. In the early days of the germ theory of disease, it was thought that only plants such as molds and bacteria could give rise to disease, but it has been further demonstrated within recent years that there are a large number of lowly organized animals which might obtain an entrance into the body and there cause disease. Historically it is of interest to note that one of the first diseases definitely proven to be associated with bacteria was a certain disease of silkworms. This was studied by Pasteur, and opened the way to subsequent discoveries of modern bacteriologists, so that at the present time it is clearly proved that such affections as diphtheria, tuberculosis, typhoid fever, pneumonia, cholera, tetanus, anthrax, relapsing fever, influenza, erysipelas, rheumatism, the plague, glanders, and actinomycosis, are due to definite bacteria, while malaria, certain forms of dysentery, certain forms of abscess of the liver, and some skin diseases are known to be caused by low forms of animal life, the protozoa; and still higher in the animal kingdom it is known that certain parasitic worms give rise to definite forms of disease, some of these being known as distomiasis, ascariasis, trichinariasis, uncinariasis, filariasis. In addition to these the ordinary tapeworms may be mentioned. With reference to the bacterial agents that cause disease, in the article on bacteria (q.v.) it was pointed out that there were a great many forms of these plants and that only a few of these were known as pathogenic, or as disease-producing. This term pathogenic, however, is purely a relative one, because under certain circumstances the introduction of well-known pathogenic micro-organisms in the human body may be harmless. In the study of the pathogenic effects of any given bacterium both the micro-organism and the animal host should be taken into consideration, because any resulting disease is a product of some detail of the characters of the infecting agent on the one hand and of the subject of the infection on the other. There may be many modifying circumstances, involving each of these factors. Thus as regards the character of the particular species of bacterium that may enter the body, its effect will depend chiefly upon its inherent virulence, the number that may be introduced into the body at one time, and the path through which the infection takes place. Thus the virulence (meaning largely the power of multiplying) of many micro-organisms in the body varies greatly under different conditions. With reference to certain number of pathogenic micro-organisms the number: The healthy tissues can resist the of given virulence, but if a larger number are introduced it may be that the animal will succumb. This is a fact demonstrated to be true for tuberculosis in high degree. The path of infection is of much importance, for if virulent micro-organisms are thrown directly into the blood-stream, and can thus traverse the entire body looking for a favorable nidus or growth, the chances are much against the organism. With reference to the subject of infection: The species of the animal, his race peculiarities, and his age are all important items. Thus it is known that tuberculosis among the Indians is extremely fatal; whooping-cough, when first introduced into New Zealand, was almost universally fatal; children are susceptible to Infectious diseases which the adult escapes; local diminished vitality plays a very important role in the production of disease in many individuals. See IMMUNITY. As to the modes of bacterial action, two main factors are known to be involved; one concerns the ability of the bacteria to multiply in the animal body after they have once entered, and the other, the production of poisons by them that may act upon the tissues. The former process is known as the infection, the latter as the intoxication. Thus in the discussion of the germ theory of disease, it is understood here that the disease process as seen in the individual is nothing more or less than an evidence of the struggle that takes place between the protective agencies of the body on the one hand and similar self-assertive processes taking place in the bacterial body, for it can hardly be denied that rapid multiplication and the production of poisons are the means emp.oyed by bacteria for self-protection. This production of poisons is one of the most interesting features of the study of bacterial diseases, and it is probably true that these poisons, or toxins as they are termed, result both from the breaking down of the nutrient materials on which the bacteria feed, and as a result of their intravital activities. It is probably true that the action of bacteria as disease-producers depends largely upon the chemical products formed directly or indirectly by them, and that this action is shown by certain tissue changes that are produced in the body, local or systemic, and by symptoms of intoxication of varying character. The tissue changes that may be produced by bacteria are so various as to include almost every type of known pathological change. In general, however, they are those of a degenerative or necrotic nature, indicating direct damage, or they are those of a reactive defensive nature on the part of the body of the infected animal, the former indicating the degree of vulnerability, the latter the protective power that the tissues possess. Many of the symptoms occurring in bacterial affections result from the changes just outlined, and it is of importance to note as a special proof of the truth of the position that bacteria may cause disease that nearly all, if not quite all, of the changes found in the organs |