240 SUSCEPTIBILITY AND IMMUNITY. 
tofore been referred to, appears to be a provision of nature for over- 
coming the infection which has already occurred. 
It has been demonstrated by experiment that naturally immune 
animals may be infected by the addition of certain substances to cul- 
tures of pathogenic bacteria. Thus Arloing was able to induce symp- 
tomatic anthrax in animals naturally immune for this disease by 
mixing with his cultures various chemical substances, such as car- 
bolic acid, pyrogallic acid, and especially lactic acid (twenty per 
cent). Leo has shown that white mice, which are not subject to 
the pathogenic action of the glanders bacillus, may be rendered sus- 
ceptible by feeding them for some time upon phloridzin, which gives 
rise to an artificial diabetes, and causes the tissues to become im- 
pregnated with sugar. 
Bouchard has found that very small doses of a pure culture of 
Bacillus pyocyaneus are fatal to rabbits when at the same time a 
considerable quantity of a filtered culture of the same bacillus is in- 
jected into a vein. The animal could have withstood the filtered 
culture alone, or the bacillus injected beneath its skin; but its resist- 
ing power—natural immunity—is overcome by the combined action 
of the living bacilli and the toxic substances contained in the filtered 
culture. The same result may be obtained by injecting sterilized 
cultures of adifferent microérganism. Thus Roger has shown that 
the rabbit, which has a natural immunity against symptomatic 
anthrax, succumbs to infection when inoculated with a culture of the 
bacillus of this disease, if at the same time it receives an injection of 
a sterilized or non-sterilized culture of Bacillus prodigiosus. Monti 
has succeeded in killing animals with old and attenuated cultures of 
Streptococcus pyogenes, or of Staphylococcus pyogenes aureus, by in- 
jecting at the same time a culture of Proteus vulgaris. In a similar 
way, it seems probable, the normal resistance of man to infection by 
certain pathogenic bacteria may be overcome. Thus when water 
contaminated by the presence of the typhoid bacillus is used for 
drinking by the residents of a certain town or district, not all of 
those who in this way are exposed to infection contract typhoid 
fever; and among those who do, there is good reason to believe that, 
in certain cases at least, the result depends upon an additional factor 
of the kind suggested by the above-mentioned experiments—e.g., the 
consumption of food containing putrefactive products, or the respi- 
ration of an atmosphere containing volatile products of putrefaction. 
The natural immunity of healthy animals may also be neutralized 
by other agencies which have a depressing effect upon the vital re- 
sisting power. Thus Nocard and Roux found by experiment that an 
attenuated culture of the anthrax bacillus, which was not fatal to 
guinea-pigs, killed these animals when injected into the muscles of 
