2 Introduction 



[2] pessimistic school in modern philosophy, was noted for his exag- 

 gerated fear of disease. 



During the greater part of the nineteenth century our knowledge 

 as to immunity has been limited to certain practical methods, often 

 efficacious it is true, but purely empirical, such as those employed in 

 immunising man against small-pox and certain domestic animals 

 against sheep-pox or pleuro-pneumonia. 



So long as the nature of the viruses was unknown no really 

 scientific study of their action or of immunity from them could be 

 made. The revelation of the organised nature of the infective viruses 

 opened up the way for these researches. This discovery, the out- 

 come of the demonstration by Pasteur of the organised nature of 

 the ferments, has enabled us to establish the part played by living 

 agents in a great number of infective diseases, and, linked with the 

 names of Davaine, Obermeyer, and above all with that of Robert 

 Koch, it has very greatly advanced the study of susceptibility and of 

 natural immunity in certain infections. 



A considerable forward step was made with the discovery, by 

 Pasteur and his collaborators Chamberland and Roux, that it was 

 possible, in certain infective diseases, to confer immunity by means of 

 micro-organisms which had had their virulence attenuated. Thanks 

 to this discovery, science was now in a position to take up the 

 thorough study of acquired immunity. The field of research was still 

 further enlarged by the demonstration of the immunising power of 

 the culture-products of pathogenic micro-organisms and above all 

 by the discovery that the blood of immunised animals is capable of 

 conferring immunity upon susceptible animals. 



Before taking up in detail the problem of immunity as it is 

 revealed to us as a consequence of these discoveries, it is essential to 

 cast a glance at infective and allied diseases as a whole and to indicate 

 in what light we look upon them in view of the present state of our 

 knowledge. 



It has been definitely established that many infective diseases 

 of man and animals are due to the invasion of small parasitic 

 organisms, sometimes of animal nature (as in itch, trichinosis, malaria, 

 Texas fever, nagana, or surra and the allied condition "dourine" in 

 horses), sometimes belonging to the vegetable kingdom like the 

 Moulds (aspergillosis), the Hyphomycetes (actinomycosis, Madura foot 

 [3] disease, bovine farcy) and the Yeasts (disease of the Daphniae, some 

 pseudomyxomas and septicaemias, pseudolupus). But by far the 



