Immniiity against any given disease may be re- 

 garded either as individually acquired or as racially 

 hereditary . 



Acquired immunity exists generally in connection 

 with those diseases, which by some as yet imperfectly 

 understood action of iheir toxins on the tissues 

 through the medium of the blood serum, actually 

 guard the individual who has had one attack from 

 having another for a greater or less period of time, this 

 period varying somewhat indefinitely according to the 

 idiosyncrasies of the disease and the individual alike. 

 Thus for a short time one attack of pneumonia confers 

 immunity against another. Small pox and its modi- 

 fication (vaccinia) confer immunity, if not actually 

 for life, still for a considerable number of jxars. 

 Measles, scarlet fever, etc. are again examples of 

 diseases which as a general rule give immunity against 

 themselves to the individual who has had the good 

 fortune to survive the first attack. ^Advantage has 

 been taken of the natural law involved in the above, and 

 immunity against certain diseases is now with more 

 or less success being sought for by means of inocu- 

 lation with serum, obtained from animals that have 

 been themselves inoculated with the attenuated virus 

 of the diseases as they originally exist. 



But this acquired immunity, however produced, is 

 onl}' partial in its benefits ; like all other characteris- 

 tics acquired during life and not inborn, it is not 

 transmissible to posterit}^ ; and, therefore, since it 

 cannot affect the life history of the race, it is only of 

 limited importance to us while regarding the subject 

 of septicaemia, except in so far as the following 

 experiment of Pasteur's bears on the subject. He 

 found that by exposing the septic bacillus (obtained 

 from cases of chicken cholera) to the air for long 

 periods, " its virulence became so reduced that 

 inoculation with the weakened or attenuated organism 



