48 ESSAYS ON BACTERIOLOGY. 



of the exhaustion of the available food-supply any 

 more tenable. It is sufficiently disproved by the fact 

 that in some diseases, which are distinctly self-limited, 

 as tetanus, diphtheria and erysipelas, the germ growth 

 is confined to a small spot in the body. And yet in 

 case of diseases in which one attack does not afford 

 protection against others, the same germs may, soon 

 after the first attack, be successfully implanted in an- 

 other part of the body, and the same phenomena of 

 self-limitation of the disease be seen. The proper 

 food is evidently not exhausted. 



Why do not the germs continue to grow indefinitely 

 in the body? Why do they die out with such regu- 

 larity? Apparently but one explanation remains. 

 There must be some actual interference with their 

 further growth, either in a temporary or permanent 

 modification of the tissiies, which renders them unfit 

 for the sustenance of that special germ, or in an agent 

 which directly antagonizes or even kills it. This is 

 the belief which is now generally and with good rea- 

 son entertained, and upon it are based the most active 

 and perhaps most promising efforts to attack the cause 

 of the infectious diseases even after they have invaded 

 the body. 



But there is another fact, long known, concerning 

 certain of the infectious diseases, equally astonishing 

 and life-saving as self-limitation, namely, self-protec- 

 tion, or immunity. It is true of some of these mal- 

 adies that one attack protects against others; title body, 



