Pathogen

“ The 'Germ Theory,' briefly stated, is this. There are all around us millions of micro-organisms, parasites which thrive and grow by feeding upon the animal world. In proportion as these parasities infest, and thrive upon a given individual, so will that individual become diseased, and it has been shown that in many cases a special germ will cause a special disease. I could deliver you a lecture, hours long, upon the classification, morphology, and pathogenic action of bacteria, but I wish at present to lead your mind into a different channel. ”

“ The method of demonstrating the germ cause of disease is as follows: Using only sterilized test-tubes, forceps, pipettes, cotton, etc., take a particle of virus, known or supposed to contain pathogenic microbes, from a person suffering from an infectious disease, insert it by a delicate glass pipette through, the sterilized cotton plug of a test-tube containing some of the prepared culture material, and deposit it there. Then place the tube in an incubator, warmed to the required degree, and let it remain for the number of hours suited to the peculiar requirement of its germ contents. ”

“ The possession of pathogenetic properties by an organism under study is indicated by the "infection" of the experimental animal—a term which is employed to summarise the condition resulting from the successful invasion of the tissues of the experimental animal by the micro-organisms inoculated and by their multiplication therein. Infection is considered to have taken place:
1. When the death of the animal is produced as a direct consequence of the inoculation. ”

“ Thirty years ago, there was no bacteriology, in the present sense of the word: and now the "habits" of these "contagia" have been studied, outside and inside the body, with amazing accuracy. It has been proved, past all possibility of doubt, that the pathogenic bacteria are the cause of infective diseases; they have fulfilled Koch's postulates—that they should be found in the diseased tissues, be cultivated outside the body, reproduce the same disease in animals, and be found again in the tissues of those animals. ”

“ The air may become infected with germs of disease from dusty trades, dried sputum, etc. If such infected air be inhaled, pathogenic271 results will follow, especially if the bacteria are present in sufficient numbers, or meet with devitalised, and therefore non-resisting, tissues. ”

“ Now we know that many diseases called infectious are caused by micro-organisms, and we group others under this class because we hold theoretically that they have their origin in microbes not yet isolated. Hence we define an infectious disease as one which is caused by a living pathogenic micro-organism, which enters the tissues from without, and is capable of multiplying therein. These micro-organisms have a time of incubation during which a poison is made in the tissues, and this brings about the intoxication we call the disease. ”

“ There are, however, some species which cause mankind much trouble by interfering in one way or another with the normal processes of life. These pathogenic bacteria, or disease germs, do not all act alike, but bring about injury to man in a number of different ways. We may recognise two different classes among them, which, however, we shall see are connected by intermediate types. These two classes are, first, the pathogenic bacteria, which are not strictly parasitic but live free in Nature; and, second, those which live as true parasites in the bodies of man or other animals. ”

“ As a consequence, the proof of the germ nature of different diseases varies all the way from absolute demonstration to mere suspicion. To give a complete and correct list of the diseases caused by bacteria, or to give a list of the bacteria species pathogenic to man, is therefore at present impossible. ”

“ Anthrax and tubercle, with perhaps the organisms of suppuration, tetanus, plague, and actinomycosis, stand in the first order of pathogenic germs. Then comes a group awaiting further confirmation. It includes the organisms related to typhoid, cholera, malaria, leprosy, diarrhœa,and pneumonia.267 Then comes in a third category, a long list of diseases, such as scarlet fever, small-pox, rabies, and others too numerous to mention, in which the nature of the causal agent is still unknown. Hence it must not be supposed that every disease has its germ, and without a germ there is no disease. ”

“ Raising the Virulence of an Organism.—If it is desired to raise or "exalt" the virulence of a feebly pathogenic organism, special methods of inoculation are necessary, carefully adjusted to the exigencies of each individual case. ”

“ Phagocytosis or the destruction of the invading organisms within the leukocytes.
4. Opsonins which render the bacteria more readily taken up by the phagocytes.
5. Enzymes other than complement possibly play a part in the destruction of some pathogenic organisms or their products. This remains to be more definitely established.
6. It is possible that in natural immunity there might be no receptors in the body cells to take up the organisms or their products, or the receptors might be present in certain cells but of a very low chemical affinity, so that combination does not occur. ”

“ But the poisonous action of most of the pathogenic bacteria is due to the chemical products secreted by the organisms, and it is to the circulation and absorption within the body of these poisons that the disturbances of the animal system, which characterise disease, decay, and dissolution of every organism, must be traced. ”

“ Some new light has been thrown upon the subject of pathogenic organisms in air by Neisser in his investigations concerning the amount and rate of air-currents necessary to convey certain species through the atmosphere. He states that the bacteria causing diphtheria, typhoid fever, plague, cholera, and pneumonia, and possibly the common Streptococcus pyogenes, are incapable of being carried by the molecules of atmospheric dust which the ordinary insensible currents of air can support, whilst Bacillus anthracis, B. pyocyaneus, and the bacillus of tubercle are capable of being aërially conveyed. ”

“ The living body has some very efficacious resistant forces which prevent most bacteria from growing within its tissues, and which in large measure may be relied upon to drive out the true pathogenic bacteria. These resistant forces are in part associated with the productions of body poisons, and are in part associated with the active powers of special cells which have been called phagocytes. ”

“ These two facts are of the utmost importance, since of all sources of infection we have the least power to guard against infection through the air, and since of all means 'of entrance we can guard the skin with the greatest difficulty. We can easily render food free from pathogenic bacteria by heating it. ”

“ Diseases which are due to unicellular pathogenic microörganisms are called infectious diseases, while if such diseases are transmitted under natural conditions from organism to organism they are spoken of as contagious diseases. ”

“ The discovery of the pathogenic role of bacteria and the development of bacteriologic science has been one of the most illuminating developments in the whole history of medicine, elucidating, as it has, the pathology of the large and important group of infectious diseases and vastly increasing the efficiency of medical and surgical treatment. ”

“ The small quantity of diseased organic matter originally introduced into the system by absorption, acts as a ferment and reproduces itself in the mass of the blood, until this becomes unfitted for the performance of its functions and the animal is killed; the active principle being thus copiously present, is exuded from the skin and lungs and gives a contagious character to the disease, or it remains only in the blood, or is secreted in pustules, constituting infection, by which the disease may be communicated to some other person. ”

“ Methods of fighting Germ Diseases.—As we have seen, diseases produced by bacteria may be caused by the bacteria being directly transferred from one person to another, or the disease may obtain a foothold in the body from food, water, or by taking them into the blood through a cut or a wound or a body opening.
It is evident that as individuals we may each do something to prevent the spread of germ diseases, especially in our homes. We may keep our bodies, especially our hands and faces, clean. ”

“ I desire now to refer again to the subject of racial and species immunity for which the humoral factors of bacterial destruction afforded an imperfect explanation, in order that I may point out that the introduction of bacteria, incapable of causing infection, into immune species is followed by immediate phagocytic ingestion and destruction of the microorganisms. The rapidity and perfection of the phagocytic reaction in insusceptible animals are very impressive and might readily lead to the decision that they suffice to explain the resistance or immunity. ”

“ For the extension of infection from one individual to another the infecting organisms must in some way be transferred. The most important of the conditions influencing this are the localization of the disease and the character of the infectious organisms, particularly with regard to their resistance to the conditions met with outside of the body. The seat of disease influences the discharge of organisms; thus, if the disease involve any of the surfaces the organisms become mingled with the secretions of the surface and are discharged with these. ”

“ There is evidence that a _mixed infection_--that is, the introduction of more than one species of organism, for example, the tubercle bacillus and a pyogenic staphylococcus--increases the severity of the resulting disease. If one of the varieties gain the ascendancy, the poisons produced by the others so devitalise the tissue cells, and diminish their power of resistance, that the virulence of the most active organisms is increased. ”

“ Those which come through the mouth come with the food or drink which we swallow; those which enter through the nose must be traced to the air; and those which enter through the skin come in most cases through contact with some infected object, such as direct contact with the body of an infected person or his clothing or some objects he has handled, etc. Occasionally, perhaps, the bacteria may get into the skin from the air, but this is certainly uncommon and confined to a few diseases. ”

“ Cellular pathology with its wonderful revelation of the anatomical seat of disease and bacteriology, with its still more wonderful revelation of the external cause of disease, so dazzled the eye and the mind that we forgot that the sensitive animal body behind the attacking microbe had its changes, too, its changes in body chemistry that could not be stated in terms of cells and bacteria. The pendulum is swinging back now to a consideration of the constitution of the body on which the microbe or poison acts, its resistance or immunity, its anaphylaxis or allergie. ”

“ These postulates have not been fully met with reference to any disease, but the principles embodied have been applied as far as possible in all those infections which we recognize as specific, and whose causative agent is accepted. In many diseases recognized as infectious and contagious no organism has been found which is regarded as the specific cause. In some of these the organism appears to be too small to be seen with the highest powers of the microscope, hence they are called “ultramicroscopic” organisms. ”

“ Many infections begin acutely and later change to the chronic type. Syphilis in man is a good illustration.
The differences between acute and chronic infections are partly due to the nature of the organism, partly to the number of organisms introduced and the point of their introduction and partly to the resistance of the animal infected.
An infectious disease is said to be specific when one kind of organism is responsible for its manifestations—as diphtheria due to the Corynebacterium diphtheriæ, lockjaw due to Clostridium tetani, Texas fever due to the Piroplasma bigeminum, etc. ”

“ Means of Invasion.—Bacteriology has shown us that different species of parasitic bacteria have different means of entering the body, and that each must enter the proper place in order to get a foothold. After we learn that typhoid infectious material must enter the mouth in order to produce the disease; that tuberculosis may find entrance through the nose in breathing, while types of blood poisoning enter only through wounds or broken skin, we learn at once fundamental facts as to the proper methods of meeting these dangers. ”

“ Modern, well-educated physicians know that many, in fact nearly all the ordinary ailments, are of germ origin. They also know, for instance, that a patient suffering from typhoid, or typhus, or some like disease, is continually reproducing these germs in immense numbers. For a physician with a full beard to lean over the bedside of such a patient is to invite millions of these germs to invade his beard, and wherever he goes he carries these germs with him and spreads the disease. ”

“ George N. Kreider, of Springfield, 111., micro-organisms enter the body first by the skin through lesions, openings of the sweat-pores, or sebaceous ducts, or by the sides of the hairs; and, secondly, by the mucous membrane through lesions of the membrane, openings of the ducts or follicles, or pockets, sulci, or folds. There are also localized infections, originating in a manner as yet unknown, and giving rise to certain violent diseases; and universal infections, giving rise to hereditary transmissible disease. ”