The facts on immunity which I have presented to you constitute the physical basis, also, of all artificial methods which are being pursued so successfully in preventing certain infections through vaccination, and in curing them through the use of immune serum products. The facts also account in an eminently satisfactory manner for the suppression of small-pox by cow-pox vaccination. The "vaccines" so-called for bacterial diseases, which are, I might say, at present being employed chiefly in protecting animals from epidemic infectious diseases to which they are much exposed, consist for the most part of bacteria either killed outright by heat or chemicals or of bacteria whose virulence has been diminished by special methods of cultivation or treatment. In human beings this method of vaccination has been employed only when large numbers of persons have been exposed to infections from the zone or focus of which they could not be removed, or from which, owing to the peculiar circumstances surrounding the infections, they could not readily or at all be protected by the suppression of the diseased germs at their sources. Thus it has been found advantageous in a few instances to employ vaccination against cholera and bubonic plague, on those especially exposed to these epidemic diseases, and against typhoid fever on troops going in time of war into heavily infected endemic zones of that disease.
In a few instances this method of vaccination has been successfully carried out in animals with infectious diseases in which the germs causing them have not been discovered. Thus it is possible to vaccinate cattle against the destructive rinderpest of Africa, the Philippines and other tropical countries, by employing the bile of animals which have succumbed to the infection, which contains the parasite of the disease somewhat modified by certain immunity principles contained within it along with parasites. In fact, this method of conjoint vaccination with the parasite of the disease and the blood containing immunity principles is one that offers a considerable field of practical application. On the one hand, there is accomplished a passive immunization of the body that becomes operative immediately and, on the other hand, a vaccination that after the usual interval leads to the production of a state of active immunity that rises to a higher level and is far more enduring than the passive state.
Incidentally we have discovered from this process of mixed or conjoint vaccination that immune sera prepared for bacteria or other parasites which are not toxin producers in the manner of the diphtheria bacillus, but which contain endotoxin, act not especially by neutralizing toxins, or by destroying outright the bacteria, but by exercising an efficient protective control over the injury which these parasites or their poisons tend to inflict on certain sensitive body cells. For example, if cattle are inoculated on one side of the body with virulent blood from animals dying of rinderpest, and on the other side with blood serum taken from animals that have recovered from the