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The result of the entrance of a given bacterium into the lishment of active immunity is given by the fact long tissues of an animal is not a disease with hard and fast established that recovery from an attack of certain infective characters, but varies greatly with circumstances. With diseases is accompanied by protection for varying periods regard to the subject of infection the chief factor is of time against a subsequent attack. Hence follows the susceptibility j with regard to the bacterium idea of producing a modified attack of the disease as a virulence is all-important. Susceptibility, as means of prevention—a principle which had been previy " is well recognized, varies much under natural ously applied in inoculation against smallpox. Immunity, conditions in different species, in different races of however, probably results from certain substances introthe same species, and amongst individuals of the same duced into the system during the disease rather than from race. It also varies with the period of life, young the disease itself; for by properly adjusted doses of the subjects being more susceptible to certain diseases, e.g., poison (in the widest sense), immunity may result without diphtheria, than adults. Further, there is the very im- any symptoms of the disease occurring. Of the chief portant factor of acquired susceptibility. It has been methods used in producing active immunity the first is whose virulence has been experimentally shown that conditions such as fatigue, by inoculation with bacteria u starvation, exposure to cold, &c., lower the general resist- diminished, i.e., with an attenuated virus.” Many of the ing powers and increase the susceptibility to bacterial earlier methods of attenuation were devised in the case of infection. So also the local powers of resistance may be the anthrax bacillus, an organism which is, however, somelowered locally by injury or depressed vitality. In this what exceptional as regards the relative stability of its way conditions formerly believed to be the causes of virulence. Many such methods consist, to speak generally, disease are now recognized as playing their .part in in growing the organism outside the body under somewhat predisposing to the action of the true causal agent, viz., unsuitable conditions, e.g., at higher temperatures than the the bacterium. In health the blood and internal tissues optimum, in the presence of weak antiseptics, &c. The are bacterium-free ■ after death they offer a most suitable virulence of many organisms, however, becomes diminished pabulum for various bacteria; but between these two when they are grown on artificial media, and the diminuextremes lie states of varying liability to infection. . The tion is sometimes accelerated by passing a current of air circumstances which alter the virulence of bacteria will be over the surface of the growth. Sometimes also the referred to again in connexion with immunity, but it virulence of a bacterium for a particular kind of animal may be stated here that, as a general rule, the virulence of becomes lessened on passing it through the body of one of an organism towards an animal is increased by sojourn in another species. Cultures of varying degree of virulence can the tissues of that animal. The increase of virulence be obtained by such methods, and immunity can be gradually becomes especially marked when the organism is inoculated increased by inoculation with vaccines of increasing virufrom animal to animal in series, the method of passage. lence. The immunity may be made to reach a very high This is chiefly to be regarded as an adaptation to sur- degree by ultimately using cultures of intensified virulence, roundings, though the fact that the less virulent members this “supervirulent” character being usually attained by the of the bacterial species will be liable to be killed off also method of passage already explained. A second method is plays a part. Conversely, the virulence tends to diminish by injection of the bacterium in the dead condition, whereby on cultivation on artificial media outside the body, espe- immunity against the living organism may be produced. Here manifestly the dose may be easily controlled, and cially in circumstances little favourable to growth. By immunity is meant non-susceptibility to a given may be gradually increased in successive inoculations. This disease, or to experimental inoculation with a given method has a wide application. A third method is by bacterium or toxin. The term must be used in injections of the separated toxins of a bacterium, the Immunity. ^ re|a^ve Sense, and account must always be resulting immunity being not only against the toxin, but, taken of the conditions present. An animal may be readily so far as present knowledge shows, also against the living susceptible to a disease on experimental inoculation, and organism. It appears to be a general law that, when an yet rarely or never suffer from it naturally, because the animal becomes immune to the toxic products of an organnecessary conditions of infection are not supplied in nature. ism, the latter ceases to be able to flourish in its tissues. In That an animal possesses natural immunity can only be the development of toxin-immunity the doses, small at first, shown on exposing it to such conditions, this being usually are gradually increased in successive inoculations ; or, as in most satisfactorily done in direct experiment. Further, the case of very active toxins, the initial injections are made there are various degrees of immunity, and in this con- with toxin modified by heat or by the addition of various nexion conditions of local or general diminished vitality chemical substances. Immunity of the same nature can be play an important part in increasing the susceptibility. acquired in the same way against snake and scorpion poisons, Animals naturally susceptible may acquire immunity, on and against certain vegetable toxins, e.g., ricin, alorin, &c. In order that the immunity may reach a high degree, the one hand by successfully passing through an attack of the disease, or, on the other hand, by various methods either the bacterium in a very virulent state or a large of inoculation. Two chief varieties of artificial immunity dose of toxin must ultimately be used in the injections. are now generally recognized, differing chiefly according to In such cases the immunity is, to speak generally, specific, the mode of production. In the first—active immunity—a i.e., applies only to the bacterium or toxin used in its reaction or series of reactions is produced in the body of production. A certain degree of non-specific immunity or the animal, usually by injections of bacteria or their increased tissue resistance may be produced locally, e.g., in products. The second—passive immunity—is produced by the peritoneum, by injections of non-pathogenic organisms, the transference of a quantity of the serum of an animal peptone, nucleic acid, and various other substances. In actively immunized to a fresh animal; the term is applied these cases the immunity is without specific character, and because there is brought into play no active change in the cannot be transferred to another animal. Lastly, in a few tissues of the second animal. The methods of active instances one organism has an antagonistic action to immunity have been practically applied in preventive another; for example, the products of B. pyocyaneus inoculation against disease; those of passive immunity have a certain protective action against B. anthracis. have given us serum therapeutics. The chief facts with This method has, however, not yielded any important practical application. regard to each may now be stated. II. Passive Immu/nity: Anti-sera.—The serum of an I. Active Immunity.—The key to the artificial estab-