40 JOURNAL AND PROCEEDINGS 



lower animal infections, ^.^., chicken cholera. The negro is less 

 susceptible to yellow fever than is the white man ; while different 

 families in a community, and even different individuals in the same 

 family, var}^ in the power of tlieir resistance to various infectious 

 diseases. Some families are strongly resistant to tuberculosis ; 

 other families are strongly susceptible. One member of a family 

 contracts every infectious disease that comes along ; another mem- 

 ber of the same family is equally exposed with impunity. Even 

 the same person may varj' greatly in his individual immunity. 

 During childhood he is particularly susceptible to children's dis- 

 eases. If he reaches adult life without having contracted measles, 

 scarlet fever, or chickenpox, he is not then very likely to become a 

 victim, even though frequently exposed to them. But, on the 

 other hand, he is now more susceptible to typhoid fever than he 

 was in childhood or than he will be in old age. Again, an in- 

 dividual may be exposed continually and directly to tuberculosis, 

 and yet his general health being good, he suffers no infection. But 

 let his system become debilitated from any cause, then, his resist- 

 ance being lessened, he readily becomes a prey to the most acci- 

 dental exposure. In these foregoing examples we have a condition 

 in which Nature has already endowed her subjects with the neces- 

 sary equipment, in whole or in part, wherewith to ward off the on- 

 slaughts of bacterial invaders. This is a condition of natural ini- 

 munit}^ and is, for the most part, inherited. When a person, 

 once susceptible to a disease, becomes non-susceptible to the same 

 disease, we say he has an acqjiired nwmnmty . We see frequent 

 daily examples of this phenomenon. An individual contracts 

 measles, mumps, scarlet fever, smallpox, typhoid fever. In the 

 great majority of cases, no matter how much he ma}- afterward be 

 exposed to these same diseases, he resists invasion. The one at- 

 tack has brought about some change in his system that renders 

 him immune to further infection, zt'.,he has acquired 2in immunit5^ 

 Now, bacteria attack an individual in different ways. In 

 diphtheria, for example, the germ first gets a foothold on the inner 

 wall of the throat. The ground being favorable, it starts to mul- 

 tipl}'. In growing, a toxin or poison is produced which, being ab- 



