GENERAL SUSCEPTIBILITY IN RELATION TO INFECTION 101 



to master these defensive forces, and accordingly becomes predisposed 

 or more susceptible to infection. 



Predisposition may be inherited or acquired. 



Inherited predisposition may be -(a) Specific, or species suscep- 

 tibility, as, e. g., dogs to distemper; cattle to contagious pleuropneu- 

 monia; hogs to hog cholera; man to gonorrhea; chancroids, acute 

 exanthemata, typhoid fever, etc. (6) Racial, as Eskimos to measles 

 and syphilis, ordinary sheep to anthrax, whereas Algerian sheep are 

 immune, etc. Racial susceptibility is frequently but a lack of acquired 

 immunity; for instance, measles, syphilis, gonorrhea, and other diseases 

 brought by settlers to foreign peoples among whom these diseases were 

 previously unknown, find them peculiarly susceptible and the diseases 

 unusually virulent, (c) Familial, i. e., members of a family may, 

 through generations, be unusually susceptible to scarlet fever, tuber- 

 culosis, rheumatism, rheumatoid arthritis, metabolic disturbances, etc. 

 (d) Individual predisposition, which depends principally upon sex, age, 

 and peculiar tissue susceptibility. Thus infants are especially prone to 

 contract certain infections on account of the immature development of 

 the body-cells, and this susceptibility to infection is further influenced 

 by acquired factors, chiefly malnutrition. On the other hand, very 

 young children enjoy an immunity to several infections, such as typhoid 

 fever, scarlet fever, and even diphtheria, probably due, as Abbott has 

 suggested, to the fact that pathogenic substances that may set up molec- 

 ular and destructive disturbances in the poorly developed cell have but 

 little effect upon the more inert protoplasm of the immature cell, and 

 that if certain bacteria gain admission to the tissues, the cells may 

 destroy them, their toxins not combining with the molecular side-chains, 

 and, as a consequence, not injuring or interfering with the cell functions. 



Acquired susceptibility bears a more important relation to infection, 

 and may be due to various factors, most of which lead to a state of re- 

 duced vitality, normal physiologic processes being impaired to a greater 

 or less degree. 



(a) Overwork or overstrain leads to general or local predisposition 

 to disease. Those engaged in hard labor, mental or physical, which 

 involves late hours and inadequate periods of rest and recreation, fre- 

 quently associated with inadequate nutrition and foul air, are likely to 

 succumb to tuberculosis, typhoid fever, pneumonia, etc. 



The influence of overstrain on acute infections has been shown ex- 

 perimentally by Charrin and Roger, 1 who found that white rats natur- 

 ally immune to anthrax became quite susceptible after being compelled 

 1 Compt. rend. Soc. de Biol. de Paris, January 24, 1890. 



