ing to elucidate is that, during a non-fatal attack of one of the specific diseases, the cellular elements implicated which do not succumb to the destructive influence of the poison acquire a tolerance to this poison which is transmissible to their progeny, and which is the reason of the exemption which the individual enjoys from future attacks of the same disease."
In my chapter on Bacteria in Infectious Diseases, in Bacteria, published in the spring of 1884, but placed in the hands of the publishers in 1883, I say: "It may be that the true explanation of the immunity afforded by a mild attack of an infectious germ disease is to be found in an acquired tolerance to the action of a chemical poison produced by the micro-organism, and consequent ability to bring the resources of Nature to bear to restrict invasion by the parasite." This theory of immunity has received considerable support from investigations made since that date, and especially from the experimental demonstration by Salmon, Roux, and others that, as suggested in the work from which I have quoted, immunity may result from the introduction into the body of a susceptible animal of the soluble products of bacterial growth—filtered cultures.
The theory of vital resistance to the toxic products evolved by pathogenic bacteria is also supported by numerous experiments which show that natural or acquired immunity may be overcome when these toxic products are introduced in excess, or when the vital resisting power of the animal has been reduced by various agencies. 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 the Streptococcus pyogenes or of Staphylococcus pyogenes aureus, by injecting at the same time a culture of Proteus vulgaris. A similar result may be obtained by subjecting animals to physical agencies which reduce the vital resisting power of the tissues. 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 the thigh after they had been bruised by mechanical violence. Charrin and Roger found that white rats, which are insusceptible to anthrax, became infected and frequently died if they were exhausted, previous to inoculation, by being compelled to turn a revolving wheel for a considerable time. Pasteur found by experiment that fowls, which have a natural immunity against anthrax, become infected and perish if they are subjected to artificial refrigeration after inoculation. This has been confirmed by the