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SUSCEPTIBILITY TO INFECTION IT has long been known that certain infectious dis

eases prevail only or principally among animals of a single species, while others are communicable to several species, including man himself. Thus typhoid fever, cholera, and relapsing fever are diseases of man, and during their epidemic prevalence none of the domestic animals contracts any of these diseases. On the other hand, the lower animals are subject to various infectious diseases which may prevail as fatal epidemics but which are never communicated to man-for example, chicken cholera, hog cholera, swine plague, rinderpest, foot-and-mouth disease. Again, several species, including man, may be susceptible to a disease while other animals have a natural immunity to it. Thus tuberculosis is common to man, to cattle, to apes, and to the small herbivorous animals (by inoculation), while the carnivora, as a rule, are immune; anthrax may be



communicated by inoculation to man, to cattle, to sheep, to guinea-pigs, rabbits, and mice, but the rat, the dog, carnivorous animals generally, and birds are immune; glanders, which is essentially a disease of horses, may be communicated to man, to the guineapig, and to field-mice, while house-mice, rabbits, cattle, and swine are to a great extent immune; smallpox is essentially a disease of man, but a modified form of the disease may prevail among cattle (cowpox). Susceptibility to infection also depends to a considerable extent upon conditions relating to the individual. It is well known that an attack of certain infectious diseases protects the individual from subsequent attacks. This subject will receive attention in the chapter devoted to “ Acquired Immunity.” But in the absence of any such acquired immunity the susceptibility of individuals of the same species differs considerably, and the same individual may be more susceptible to infection at one time than at another. Certain families or races are especially susceptible to infection by certain disease germs. Thus the negro race is less susceptible to yellow fever and to the malarial fevers than the white race; on the other hand, smallpox is exceptionally fatal among negroes and dark-skinned races. In general it may be said that when an infectious disease is first introduced among primitive races, who, by reason of their isolation, have been previously exempt from it, it is apt to be exceptionally fatal. This is no doubt due to the fact that there has been no opportunity for the operation of the laws of natural selection, by “survival of the fittest.” But under the operation of these laws, in process of time, a certain degree of race immunity is likely to be established.

Individual susceptibility depends to some extent upon age. As a rule, young animals are more susceptible to infection by inoculation than adults of the same species. In the human race we recognise certain diseases as especially liable to prevail among children-scarlet fever, whooping-cough, etc. It is also known that the tendency to tubercular infection diminishes with advancing years. Tubercular meningitis and tubercular joint diseases are most common in children, and pulmonary.consumption in young adults. Again, the susceptibility of individuals depends to a considerable extent upon conditions relating to their general health. Various depressing agencies increase the susceptibility to infection. Most prominent among these are insufficient food, insanitary surroundings, great fatigue, and mental worrygrief, fear, etc. The fact that pestilence and famine are likely to go hand in hand has long been known. Whether the prevailing epidemic be cholera, bubonic plague, relapsing fever, typhus, or smallpox, the influence of insufficient food is most marked, and in times of distress, due to failure of the food supply, any infectious disease is liable to exhibit a malignancy and fatal character, although under ordinary conditions it may be comparatively harmless. Insanitary surroundings, by vitiating the air, insufficient ventilation and overcrowding of dwellings, factories, schoolhouses, etc.—all have a tendency to lower the vital resisting power of individuals subjected to such influences and to increase the susceptibility to infection. Debility resulting from loss of blood or the exhaustion following great fatigue also increases the susceptibility to various infectious diseases. Clinical observation shows that a similar result follows the excessive use of alcoholic drinks.

Localised infectious processes are not only more liable to be established in individuals whose vital energy is reduced by any of the causes mentioned, but also as a result of causes which reduce the resisting power of the tissues at the point of invasion. Thus a carbuncle or an abscess may develop in tissues that have been bruised or injured in any way; and the congestion or inflammation of the fauces which is so common as a chronic or acute condition

—a “sore throat”-no doubt increases the susceptibility to diphtheritic infection in this locality. It is well known that pneumonia often follows attacks of

measles, of influenza, or of bronchitis, in which diseases there is a catarrhal inflammation of the bronchial tubes which appears to favour infection by the specific micrococcus which is the usual cause of pneumonia. The victims of chronic alcoholism are especially subject to pneumonia.

Certain occupations increase the susceptibility to certain infectious diseases. Thus pulmonary consumption is more likely to be developed in those who lead an indoor life, whose occupation has a tendency to prevent full expansion of the lungs—tailors, seamstresses; and in persons who are compelled to breathe a dust-laden atmosphere-factory hands, grinders, etc. Susceptibility to pneumonia, influenza, tonsillitis, and diseases of the air passages generally is increased by living in over-heated apartments. Exposure to cold, per se, is not likely to increase the susceptibility to such infections in individuals who are habituated to living in the open air, such as sailors, hunters, soldiers living in tents, etc.

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