1 18 IMMUNITY AND INFECTION 
for example, strains of a given organism might produce mild disease 
in areas where it has been endemic for generations and yet be rapidly 
fatal for alien populations who may have in turn become partly 
tolerant for other strains of the same organism.^ If such prove to be 
the case, unrestricted immigration may lead to tempprary disturbances 
in the balance between specific microorganisms on the one hand and 
hosts on the other— a feature to which Theobald Smith called attention 
many years ago.^ 
Racial differences in susceptibility are occasionally met with even 
in the same species. Negroes and Indians are more susceptible to 
infection with the tubercle bacillus than the Caucasian race. The Jews 
appear to be somewhat more resistant to infection with the tubercle 
bacillus than the other branches of the Caucasian race. 
n. INFECTION. 
Pathogenic bacteria which reach the host do not necessarily incite 
disease; they may be, and undoubtedly are, frequently overcome by 
the body without inducing symptoms. This primary resistance to infec- 
tion involves an initial struggle between host and microorganism which 
brings into play non-specific lines of defense of the macroorganism 
consisting collectively of the skin, mucous membranes of the respira- 
tory and gastro-intestinal tracts and other intact barriers discussed 
in the preceding chapter. If this initial line of defense holds, the host 
overcomes the prospective invader at the frontier, and the latter fre- 
quently perishes. Repeated microbic assaults may be successful even if 
the first fails. On the other hand, if the microbe prevails and pene- 
trates the initial line of defense, invasion of the tissues of the host occurs 
and the microorganism encounters a second line of defense which is 
made up of tw^o rather distinct but non-specific factors— cellular and 
humoral. The cellular defense of the host resides in the leukocytes 
which circulate in the body fluids and in certain fixed tissue cells in the 
lungs, lymph spaces and glands, as the stellate or Kupft'er cells of the 
liver, as well as large cells which are found in serous cavities. These 
cells engulf and destroy certain types of invading microorganisms. The 
humoral defense resides in the natural, non-specific power of the blood 
and lymph to destroy limited numbers of microorganisms or to so inter- 
fere with their nutrition or other functions as to prevent their devel- 
opment within the body. The normal humoral defense is frequently 
effective against bacteria which do not succumb to the cellular defense 
of the body, and vice rersa. 
It is recognized that certain environmental factors predispose to 
infection. Thus, extreme climates, excessive humidity, or exposure 
to unhygienic conditions, bad air, poor or insufficient food, lack of 
exercise or fatigue may react upon the individual in ways not definitely 
1 See section on Balanced Parasitism and Epidemiology, page 109. 
2 Smith, Theobald: Trans. Assn. Am. Phys., 1894. 
