IMMUNITY 3 



necessary, therefore, that bacteria, in order to incite 

 disease, should belong strictly or facultatively to the 

 class known as parasitic. It must not be forgotten, 

 however, that the terms are relative, and that bacteria 

 ordinarily saprophytic may develop parasitic and 

 pathogenic powers when the resistant forces of the 

 invaded subject are reduced to a minimum by chronic 

 constitutional disease or other cause. Organisms 

 that are parasitic, however, are not necessarily patho- 

 genic, and there are certain more or less fundamental 

 requirements which experience has taught us must be 

 met by the organism in order that it may be infectious 

 for any given animal ; and by infectiousness is meant 

 the ability of an organism to live and multiply in the 

 animal fluids and tissues* Variations in virulence 

 occur, not only among different species of pathogenic 

 organisms, but among bacteria of the same species. 

 It is necessary, therefore, in order that infection may 

 occur, that the particular organism invaded shall 

 possess sufQcient virulence. 



Whether or not infection occurs depends also upon 

 the number of organisms that gain entrance to the 

 animal tissues. Bacteria, therefore, must be in suffi- 

 cient number to overcome local defences and to gain 

 a definite foothold and carry on their life processes 

 before they can give rise to infection. 



The Path of Infection.— The portal by which 

 bacteria gain entrance is of great importance in 

 determining whether disease shall occur or not. 

 Streptococci when swallowed may be entirely in- 



