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 beginning 

 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 infection, 

 and the terms infection and infectious disease were practically used 

 synonymously. This conception of the terms seemed quite warrant- 

 able at the time in view of the findings in such a disease as tuber- 

 culosis, 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 future investiga- 

 tions have shown to be erroneous. For it was soon demonstrated 

 that pathogenic organisms may be present on the tegumentary or 

 mucous surfaces of the body without concomitant 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. Streptococci 

 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 of bacteria, the 

 majority of them harmless saprophytes, but in addition there are 



