112 IMMUNITY AND INFECTION 



hunger, experimental (phloridzin) diabetes, fatigue produced by pro- 

 longed exercise in treadmills, and excessive chilling by the removal 

 of hair have been shown to decrease resistance in experimental animals. 



It is also a matter of common observation that the uniform exposure 

 of a number of theoretically susceptible individuals of the same species 

 to a virus does not lead to uniform infection; a certain small number 

 usually perish, a larger proportion become mildly or severely ill and 

 recover. The greatest number are not especially affected, as a rule. 

 Those individuals who escape infection in one epidemic may succumb 

 to infection during a subsequent epidemic of the same disease. This 

 phenomenon of individual variation in susceptibility is well exempli- 

 fied in water- and milk-borne epidemics of typhoid fever where typhoid 

 bacilli are widely distributed in a water or milk supply. A small 

 number become infected, but the greater number escape the disease. 

 The incidence of scarlet fever, of diphtheria or of other infectious 

 diseases among the members of the same family frequently illustrates 

 this same phenomenon. This resistance to infection exhibited by 

 certain individuals of a susceptible species is termed inherited 

 immunity. 



Susceptible individuals who survive a naturally acquired or arti- 

 ficially induced infection as smallpox, measles, typhoid fever or 

 vaccinia are frequently resistant or refractory to subsequent infec- 

 tion with the same virus. They have developed a resistance to specific 

 infection, they have acquired immunity, in other words. This type 

 of immunity, which results from actual infection, is termed active, 

 acquired immunity. It is the outcome of a successful struggle between 

 the host and the invading microbe during which the former, through 

 cellular activity, produces or increases antibodies specifically inimical 

 to the latter. The immunity which is thus laboriously produced 

 is frequently fairly persistent. It is more commonly observed fol- 

 lowing invasion by exogenous, progressively pathogenic bacteria than 

 infection with endogenous microorganisms of the "opportunist" type. 

 Indeed, infection with the latter not infrequently results in increased 

 susceptibility to subsequent infection with the same species of microbe. 

 Thus, recovery from one attack of typhoid fever usually confers last- 

 ing immunity upon the individual; one attack of lobar pneumonia, 

 on the other hand, appears to predispose the individual to subsequent 

 infection with the pneumococcus. 



The injection of specific immune substances or antibodies into 

 susceptible individuals may confer upon them transient or temporary 



