20 INTRODUCTION 



problem of the infectious diseases. How does infection primarily 

 take place? how does infection give rise to disease? and how does 

 the animal body overcome infection? these are the most important 

 questions which at present occupy the attention of immunologists 

 the world over, and it is the object of the present work to present 

 to the practising physician the more important data which have 

 already been worked out. 



Infection and Infectious Disease. In the earliest days of bacteri- 

 ology, when the pathogenic role of various bacteria was just be- 

 ginning to be understood, it was thought that the presence of such 

 organisms in the animal body could only be anticipated if symptoms 

 of the corresponding disease existed at the same time, or were about 

 to appear; in other words, the presence of a pathogenic bacterium 

 in the body of an individual was looked upon as equivalent to in- 

 fection, and the terms infection and infectious disease were prac- 

 tically used synonymously. This conception of the terms seemed 

 quite warrantable at the time, in view of the findings in such a dis- 

 ease as tuberculosis, where it had just been established that the 

 disease in question was invariably associated with the presence of 

 the tubercle bacillus, while the existence of the tubercle bacillus in the 

 body in the absence of a corresponding lesion was unknown. The 

 majority of physicians hence readily accepted this line of thought, 

 which further investigations have shown to be erroneous. For it 4 

 was soon demonstrated that pathogenic organisms may be present 

 on the tegumentary and mucous surfaces of the body without con- 

 comitant disease. 



It is thus well known that staphylococci are present at almost 

 any point of the skin, and that streptococci even may here be demon- 

 strated in perfectly healthy individuals. Pneumococci may be 

 found in the mouths of almost every individual, non-virulent to be 

 sure, in the majority of people, but virulent in fully 15 to 20 per 

 cent, of the cases, in the absence of any symptoms of disease. Strep- 

 tococci are here likewise not infrequent. Tubercle bacilli have been 

 found in the nasal secretion of healthy attendants on tubercular 

 patients. Diphtheria bacilli are frequently encountered in those 

 who have been about diphtheria patients, and normal carriers of / 

 the meningococcus, in districts in which the corresponding disease 

 is prevalent, are frequently more common than patients with the 

 disease. Then, in the normal intestinal contents there are myriads 



