reproduction, increases in a geometrical proportion, so that in a comparatively 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, infectious 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 concluded 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.
Recent experiments indicate that specific ferments can only dissociate 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-
