Virulence

“ The great novelty which is the outcome of the preceding facts, and which gives them a distinct place in our knowledge of virulent diseases, is that we have here to do with a disease of which the virulent agent is a microscopic parasite, a living organism cultivated outside of the animal body, and that the attenuation of the virulence is in the power of the experimenter. ”

“ The sources from which the infection is derived he believed to be—1st, from the dead body, regardless of age, sex, or disease, no matter whether the latter is of puerperal or non-puerperal origin, the virulence depending upon the stage of decomposition; 2d, diseased persons, whose malady is associated with decomposition of animal tissue, no matter whether the affected person suffers from childbed fever or not, the decomposing matter alone furnishing the product from which infection is derived ”

“ It is a common experience to see a malady transformed through the effects of heredity or acquired immunity, through environment or the temporary mitigation of virulence in the germ; and again we see the same disease, no longer restrained by such inhibitory conditions, bursting forth as a malignant and deadly plague. We have, therefore, no warrant for the hypothesis that a pathogenic germ which, under given conditions of life, has lost in pathogenesis, but not in vitality, should continue forever to exist as a harmless microbe. ”

“ If these germs, however, are conveyed to another person, who has never had the disease, or whose tissue is not immune, they will immediately resume their full activity and virulence, and will establish the disease, frequently in its most violent form, in the person so infected. ”

“ The early inception of the malady and its insidious progress, with the symptoms and diseases resulting, easily deceives the victim as well as the parents and medical advisers, until the long-pent-up virulence breaks forth, showing itself in every part of the tabernacle of the spirit of man, when the removal of the primary cause does little or no good. ”

“ 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. ”

“ On the other hand, he found the weakened virus could be again strengthened by inoculating a feeble animal such as a guinea-pig a day or two old with it, and then inoculating stronger and stronger animals: an increase in strength was gained with each inoculation, until at last the virus could attack the strongest. A similar increase in virulence appears to occur in plague, where animals, especially rats and mice, seem to be affected before human beings, and not only increase the virulence of the microbes, but convey the infection. ”

“ Of the chief methods used in producing active immunity the first is by inoculation with bacteria whose virulence has been diminished, i.e. with an “attenuated virus.” Many of the earlier methods of attenuation were devised in the case of the anthrax bacillus, an organism which is, however, somewhat exceptional as regards the relative stability of its virulence. Many such methods consist, to speak generally, in growing the organism outside the body under somewhat unsuitable conditions, e.g. at higher temperatures than the optimum, in the presence of weak antiseptics, &c. ”

“ In the two former examples, Thrale pointed out, and in many other diseases, infectious or contagious, or both, these pestilences had gradually lost virulence. By the elimination of those most susceptible to infection and incapable to resist the onslaught of the disease, and by the survival of those whose vitality was strong enough either to resist attack or to achieve recovery, mankind at last were gradually becoming immune against certain infections which had prevailed in the past. ”

“ And again he says that, "under certain conditions, human excrementitious matter in certain diseases is almost certain poison, producing the parent-disease in great numbers of cases of those exposed to it, with a degree of virulence proportional to its concentration." ”

“ By allowing the organism to grow in a well-aerated culture the virulence is gradually diminished, and this virulence, once abolished, does not return even when numerous successive cultures are made on agar or other nutrient media. On the other hand, by passing the cholera bacillus successively through the peritoneal cavities of a series of about thirty guinea-pigs, he obtains a virus of great activity ”

“ Almost invariably it will travel on to another locality, carried thither by human intercourse, and will there build again a local epidemiologic picture more or less modified by changes in the environment and changes in the virulence of the virus itself. ”

“ We know there is a great deal of variation in the virulence of the different pathogenic organisms, and the virulence can be artificially increased and diminished. In epidemics of meningitis the virulence of the organisms is increased, as is shown by the greater mortality. It is highly probable that such epidemics are due to changes which arise in the organisms from causes we do not know and which increase their capacity for harm. It is possible that such a change would convert a carrier into a case of disease, the organism acquiring greater powers of invasion. ”

“ 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. ”

“ There can be no doubt that different epidemics possess different degrees of virulence, and what proves a sufficient protection in a [Pg 47] mild epidemic of small-pox may not be sufficient in a more virulent one. In certain seasons and in certain conditions of the atmosphere, the human system is more prone to certain disease than at other times. ”

“ The proof that for at least some diseases, the conception of spontaneous virulence must be forever abandoned—as well as the idea of contagion and an infectious element suddenly originating in the bodies of men or animals and able to originate diseases which propagate themselves under identical forms: and all of those opinions fatal to medical progress, which have given rise to the gratuitous hypotheses of spontaneous generation, of albuminoid ferments, of hemiorganisms, of archebiosis, and many other conceptions without the least basis in observation. ”

“ Probably both elements are active; as the relative immunity of the host rises the infectivity or virulence of the parasite must rise to an equal extent to maintain the equilibrium. If this were true we would find that in those localities in which the disease is endemic and where the population is relatively resistant there is a normally more virulent virus in existence. A stranger coming into such a community would, in view of his lower resistance to the virus, be more susceptible of becoming actively infected. ”

“ The virus of a cursing creed is rendered comparatively harmless by the time it reaches the young sinner in the nursery. Its effects fall as far short of what might have been expected from its virulence as the pearly vaccine vesicle falls short of the terrors of the confluent small-pox. ”

“ For the chief symptoms of onset see page 318.
Persons vary in susceptibility to attack by different infective diseases. The intensity of an attack depends on the condition of the patient, and on the number and the virulence of the particular microbes infecting the patient. In certain families attacks of particular diseases are more severe, and attacks are more liable to occur than in others.
It has been shown in certain diseases that the cells and the fluids of the body have a protective effect against infection. ”

“ It has also been shown that the various microscopic organisms, which are believed to be the specific cause of certain diseases, increase in virulence, when they propagate in the absence or with a deficient supply of oxygen, while the intensity of their action is diminished if they grow where oxygen is plentiful. ”

“ After some time the disease loses its virulence, for reasons that we cannot as yet explain, though it continues to carry off {80} a regular, if small, number of victims, and then it may begin to rage again as destructively as before. ”

“ The air about poor patients is more infectious than about the rich. These things being established, I am of opinion that, in the greatest contagion, we may securely see a patient at the distance of two ells, if the chamber windows be not all shut.——Fra. Luigi: The infection is greater or less in proportion to the virulence of the contagion; but I have made no observation as to the distance. The disease is communicated by contact of all infected things, and by close inspiration of the breath of the sick.
4. What are the seasons in which the plague chiefly appears ”

“ But it is obvious that in the majority of cases it must be impossible to decide by which of these channels the disease is conveyed, from the nature of the intercourse between the physician and the patient.
3. It is not pretended that the contagion of puerperal fever must always be followed by the disease. It is true of all contagious diseases, that they frequently spare those who appear to be fully submitted to their influence. Even the vaccine virus, fresh from the subject, fails every day to produce its legitimate effect, though every precaution is taken to insure its action. ”

“ We have now reached a phase of this question when it is of the very last importance that light should once for all be thrown upon the manner in which contagious and infectious diseases take root and spread. To this end the action of various ferments upon the organs and tissues of the living body must be studied; the habitat of each special organism concerned in the production of each specific disease must be determined, and the mode by which its germs are spread abroad as sources of further infection. ”

“ The disease bears as constant a relation to its contagium as the microscopic organisms just enumerated do to their germs, or indeed as a thistle does to its seed. No wonder, then, with analogies so obvious and so striking, that the conviction is spreading and growing daily in strength that reproductive parasitic life is at the root of epidemic disease—that living ferments finding lodgment in the body increase there and multiply, directly ruining the tissue on which they subsist, or destroying life indirectly by the generation of poisonous compounds within the body. ”

“ They gain an entrance through the lungs, food canal, or skin, and, living upon the fluids and tissues, multiply with great rapidity until they permeate the entire body. Not only do they destroy the protoplasm, but they form waste products, called toxins, which act as poisons. Diseases caused by germs are known as infectious, or contagious, diseases.129 The list is a long one and includes smallpox, measles, diphtheria, scarlet fever, typhoid fever, tuberculosis, la grippe, malaria, yellow fever, and others of common occurrence. ”

“ I have said that our knowledge in this direction has, even in the last two or three years, made enormous strides, and it is now almost a matter of certainty that all contagious or infectious disorders, as well as many others, are but the expression of the fact that minute living bodies have made a resting-place for themselves in or upon other living tissues; and that the development of the phenomena of these morbid states is but an indication of their presence and reproductive activity—either as cause, or as an accompaniment of these disease manifestations. ”

“ Among the infectious diseases known to be due to specific germs are the following: typhoid, typhus, relapsing fevers, cholera, diphtheria, croup, tuberculosis, pneumonia, malaria, yellow fever, erysipelas, septicæmia, anthrax, tetanus, gonorrhea, etc.; and among the infectious diseases the germs of which have not as yet been discovered are the following: scarlet fever, measles, smallpox, syphilis, varicella, etc.
The part of the body and the organs in which the [Pg 239] germs first find their entrance, or which they specifically attack, vary with each disease ”

“ 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. ”

“ Remember that we and the disease germs are both cells; and that, if they are numbered by millions, we are by billions; and that we are made up of far the older and the tougher cells of the two. Except in a few of the most virulent and deadly of fevers, like the famous "Black Death," or bubonic plague, and lock-jaw, or tetanus, ninety-five times out of a hundred when disease germs get into our bodies, it is our bodies that eat up the germs instead of the germs our bodies. Keep away from disease germs all that you reasonably and possibly can ”

“ The possibility of such a method was suggested by the familiar observation, made by Pasteur and numerous other workers, that animals of different species differ widely in their susceptibility to various maladies, and that the virus of a given disease may become more and more virulent when passed through the systems of successive individuals of one species, and, contrariwise, less and less virulent when passed through the systems of successive individuals of another species. ”