INFECTIOUS DISEASES. 73 



reproduction, increases in a geometrical proportion, so that in a com- 

 paratively short time the whole mass is attacked. Thus a small amount 

 of yeast will start the dissociation of thousands of gallons of a mash — 

 the bacteria, in a septic tank, derived from the human digestive tracts, 

 liquefy in twelve hours all the sewage of a household, including the 

 paper and other cellulose matter. Pathogenic germs, in media such as 

 nutrient gelatine, agar-agar, blood serum, etc., also develop quite 

 rapidly ; nevertheless the progress of infectious diseases is comparatively 

 slow and frequently localized. 



Since many pathogenic germs are present in the air we breathe, 

 in the water we drink, and in the soil we come in contact with, infec- 

 tious diseases seemingly ought constantly prevail; this not being the 

 case, it has been assumed that the system itself under normal conditions 

 can withstand their attack. Predisposition, on the other hand, betokens 

 a weakened system, i. e., decreased vitality, or one in which the way for 

 the entrance of the germs has been paved. From this it may be con- 

 cluded that the so-called specific disease germs generally present are no 

 more dangerous than the predisposing media. Immunity can be 

 obtained either by avoiding the pathogenic germs or the predisposing 

 cause. At present means are suggested to isolate the consumptive and 

 destroy the intermediate-bearing host, both in malaria and yellow 

 fever. It is not even known whether other carriers equally dangerous 

 exist. 



The destruction of the predisposing cause certainly is more humane, 

 and the one to be sought for. Unfortunately success has been limited to 

 but few of the many diseases known to be of an infectious nature. 

 Probably failure is largely due to ignorance of causes producing 

 immunity, and in fact every theory so far advanced has been badly 

 battered by numerous failures of substantiation. 



Eecent experiments indicate that specific ferments can only dis- 

 sociate specific foods, i. e., the assimilable ones for any one class of 

 ferments do not constitute a wide range. Furthermore, it has been 

 proved that micro-organisms can either be antagonistic to each other, 

 i. e., one can change some nutrient essential to the other's existence, or 

 synergetic, where one is dependent upon the other for its food — and, 

 in some cases, they may be indifferent where the food of the one is not 

 that of the other. 



Synergetic action plainly shows how limited is the dissociating 

 power of micro-organisms. Thus an albuminoid is first converted into 

 an ammonia compound by one class of ferments, a second class produces 

 nitrites and a third nitrates. In the absence of the first, neither 

 nitrites nor nitrates can be formed, and in the absence of the second no 

 nitrate will appear. Still more striking — most exact experiments have 

 shown that both ferments and enzymes cannot assimilate food chem- 



