CHAPTER XI NATURAL IMMUNITY IT is hardly necessary to explain that absence of susceptibility to an infectious disease constitutes what is known as immunity for or against the disease in question. Now this immunity may be natural or acquired—that is, due to inheritance or developed in a susceptible individual subsequent to birth. We have said in a preceding chapter that man is immune as regards certain infectious diseases of the lower animals, and that many of the infectious diseases to which he is subject are not transmitted to the domestic animals with which he is most closely associated. This natural immunity is not, however, in all cases absolute and complete. For example, the white rat possesses a remarkable immunity against anthrax, a disease which may be communicated by inoculation to sheep, cattle, rabbits, guinea-pigs, mice, and to man himself. But it has been shown that this natural immunity of the white rat may be overcome by giving it an exclusively vegetable diet. Again, natural immunity may in some cases be overcome by the devitalising agencies mentioned in the chapter on susceptibility to infection (starvation, great fatigue, etc.). Infection also depends upon the comparative virulence of the infecting agent, or germ, and to some extent upon the number of germs introduced. Immunity, therefore, whether natural or acquired, often has only a relative value, and may be overcome as a result of circumstances favourable to infection. Thus it has been found that germs having very little pathogenic virulence, and harmless under ordinary conditions, may kill guinea-pigs when injected into the muscles of the thigh after they have been bruised by mechanical violence. Pasteur found that fowls, which have a natural immunity against anthrax, become infected and die if they are subjected to artificial refrigeration after inoculation. Pigeons have a natural immunity against anthrax, but if they are enfeebled by lack of food they succumb to inoculations with the anthrax bacillus. The pathogenic power of known disease germs also varies greatly as a result of conditions relating to their development. In general it may be said that cultivation in the bodies of susceptible animals increases the virulence of disease germs. Attenuation of virulence may be effected by several methods, all of which depend upon subjecting the germs to prejudicial influences of one kind or another-long exposure to oxygen, exposure to a temperature a little short of that which would completely destroy their vitality, exposure to various chemical agents. Attenuated germs may cause infection in very susceptible animals, and may gain in virulence as a result of their growth in such animals. After passing through a series of susceptible animals they may finally acquire such pathogenic virulence that they can overcome the resisting power of animals having a considerable degree of natural immunity. Applying the facts ascertained by experiments upon the lower animals, we can understand how the earlier cases in an epidemic may occur in the most susceptible individuals, and are often comparatively mild; but, as a result of its transmission through a series of individuals, the germ gradually increases in virulence and the epidemic in malignancy. Thus the earlier cases in an epidemic of diphtheria or of scarlet fever are often mild, while later cases prove to be extremely difficult to manage and show a high rate of mortality. Infection also depends to some extent upon the number of germs introduced. The resources of nature, upon which immunity depends, may be sufficient to dispose of a few typhoid bacilli or diphtheria bacilli; while a larger number introduced at one time may overwhelm the resisting power of the individual. The essential difference between a susceptible and immune animal depends upon the fact that in one the pathogenic germ, when introduced by accident or experimental inoculation, multiplies and invades the tissues or the blood, where, by reason of its nutritive requirements and toxic products, it produces changes in the tissues and fluids of the body which constitute disease and may result in death. On the other hand, in an immune animal multiplication of the germ and consequent disturbance of vital functions does not occur, or is restricted to a local invasion of limited extent, in which the parasitic invader soon succumbs to the resources of nature. This essential difference evidently depends upon conditions favourable or unfavourable to the development of the germ; or upon its destruction by some active agent present in the tissues or fluids of the body of the immune individual; or upon a neutralisation of its toxic products by some substance in the body of the animal which resists infection. Among the unfavourable conditions which may be supposed to prevent the development of disease germs in animals which have a natural immunity against infection by them, we may mention, first, the temperature of the animal. It is well known that the constant body temperature of mammals varies considerably for different species. Birds, as a rule, have a higher temperature than mammals, and reptiles are "cold-blooded animals." A disease germ, like the tubercle bacillus, for example, which requires for its development a temperature not very different from that of a healthy man, may fail to infect a pigeon because of its comparatively high, or a frog because of its low, temperature. Certain experiments which have been made by bacteriologists give support to this view. This is the explanation offered by Pasteur of the immunity of fowls against anthrax - a disease of sheep and cattle; and in support of this view he showed by experiment that when chickens are refrigerated by being immersed in cold water, after inoculation, they are liable to become infected and to die. Again, the composition and especially the reaction of the blood and other body fluids may perhaps be the determining factor. Some germs do not grow readily in an alkaline medium; and some animals-for example, the white rat — have a highly alkaline blood. Experiments made by the German bacteriologist, Behring, seem to show that the natural immunity of the white rat against anthrax infection is lost when the animal is given food which reduces the alkalinity of its blood. It is probable, also, that the presence or absence of |