192 INFECTION AND IMMUNITY 



infection, provided the mode of inoculation has been the same and the 

 virulence of the germ not excessive. It would seem, therefore, that 

 among these animals the difference in resistance in the face of an arti- 

 ficial infection between individuals of the same race is very slight. 



In higher animals, however, especially in the case of man, the ex- 

 istence of such apparent individual differences is a well-established fact, 

 although in judging of them we must not forget that the conditions of 

 infection are not subject to the uniformity and control which animal 

 experimentation permits. Of a number of persons exposed to any 

 given infection there are always some who are entirely unaffected and 

 there are great variations in the severity of the disease in those who 

 are attacked. In the absence of positive evidence in support of the 

 direct inheritance of this individual immunity, the most reasonable 

 explanation for such differences in resistance seems to lie in attrib- 

 uting them to individual variations in metabolism or body chem- 

 istry. Depressions, for instance, in the acidity of the gastric secretion 

 would predispose to certain infections of gastro-intestinal origin. Ana- 

 tomical differences, too, may possibly influence resistance. Thus, 

 Birch-Hirschfeld believed that certain anomalous arrangements of the 

 bronchial tubes predisposed to tuberculosis. 



Instances of transient susceptibility induced by physical or mental 

 overwork, starvation, etc., should hardly be classified under this head- 

 ing, since the conditions in such cases correspond simply to experi- 

 mental depression of natural species or race resistance. 



Acquired Immunity. — It is a matter of common experience that many 

 of the infectious diseases occur but once in the same individual. This 

 is notably the case with typhoid fever, yellow fever, and most of the 

 exanthemata, and is too general an observation to require extensive 

 illustration. A single attack of any of the diseases of this class alters in 

 some way the resistance of the individual so that further exposure to 

 the infective agent is usually without menace, either for a limited period 

 after the attack, or for life. Resistance acquired in this way is often 

 spoken of as acquired immunity. 



The protection conferred by certain diseases against further attack 

 was recognized many centuries ago, and there are records which show 

 that attempts were made in ancient China and India to inoculate healthy 

 individuals with pus from small-pox pustules in the hope of producing 

 by this process a mild form of the disease and its consequent immunity. 



Pasteur, before all others, thought philosophically about the phenom- 

 ena of acquired immunity, and, with adequate knowledge, realized the 



