1 8 THE POPULAR SCIENCE MONTHLY 



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 sup- 

 pression 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 dis- 

 eases 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 suppres- 

 sion 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 dis- 

 eases, 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 applica- 

 tion. 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 con- 

 joint vaccination that immune sera prepared for bacteria or other 

 parasites which are not toxin producers in the manner of the diph- 

 theria 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 



