INFECTION AND IMMUNITY 337 



appear as though among the individual bacteria com- 

 prising the culture some were pathogenic and some 

 vegetative. When the organisms are frequently trans- 

 planted from culture medium to culture medium, the 

 vegetative individuals thrive best and eventually alone 

 survive, the others having been outgrown and outlived, 

 after which no virulence can be revived. When, on the 

 other hand, they are frequently passed through animals 

 the pathogenic individuals thrive and the vegetative 

 ones are eliminated. 



The avenue by which the microparasites enter the host 

 seems to be of importance. Certain species thrive only 

 when taken into the alimentary organs; others only 

 when applied to the mucous membranes; still others 

 only when taken into the respiratory organs. The 

 avenue of entrance also determines the form that an 

 infection may take. Thus streptococci, minute spheri- 

 cal organisms, hanging together like a string of beads, 

 may cause erysipelas if entering through the skin; sore 

 throat with the formation of a false membrane if taken 

 into the mouth; abscesses if carried into the deeper 

 tissues; puerperal fever if introduced into the uterus, and 

 disease of the valves of the heart if into the circulation. 



The host must be in a receptive condition. This is one 

 of the most interesting of all the cardinal conditions, 

 implying as it does some variation in the physiologico- 

 chemical condition of the host by which the invasive 

 power of the microparasites is made possible or im- 

 possible. When the host is receptive it is described as 

 susceptible; when resistant, as immune. 



Immunity may be defined as a physiologico-chemical" 

 condition of the host by which its invasion by micro- 

 parasites is made impossible. When one inquires the 

 nature of this physiologico-chemical condition by which 

 the invasion by micro-parasites is made impossible, he 

 enters upon an investigation the scope and complexity 

 of which seem to increase as the subject is pursued. 

 Microparasites eliminate "aggressins" tending to de- 



22 



