GENERAL PHENOMENA OF IMMUNITY 115 
usually perish, a larger proportion become mildly or severely ill and 
recover. The greatest number are not especially affected, as a rule. 
Those indi^•idllals 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 individual 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 w^hich is thus laboriously produced is 
frequently fairly persistent. It is far more commonly observed fol- 
lowing invasion of exogenous, progressively pathogenic bacteria than in 
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. 
This distinction is very important when viewed in the light of current 
theories of immunity. (See p. 132.) Thus, recovery from one attack 
of typhoid fever usually confers lasting immunity upon the individual; 
one attack of furunculosis, or of pneumonia, on the other hand, appears 
to predispose the individual to subsequent infection with the staphylo- 
coccus, or invasion by one or another of the pneumococcus types. 
The injection of specific immune substances or antibodies into 
susceptible indi\'iduals may confer upon them transient or temporary 
immunity or increased resistance to the specific infection; the host is 
a passi^'e recipient of antibodies in such instances. These alien anti- 
bodies, however, soon diminish in potency or disappear, lea\ing 
the susceptibility of the individual to infection at its original level. 
Immunity induced by the injection of specific antibodies is termed 
passive acffuired immunity. The transitory immunity to diphtheria 
or tetanus following the injection of diphtheria or tetanus antitoxin is 
an example of passive acquired immimity. 
Immunity may be localized or general in the same indi^•idual, and 
different individuals frequently exhibit varying degrees of resistance 
or susceptibility to the same virus. 
