INTRODUCTORY AND GENERAL 5 



occur repeatedly and at short intervals in the same person. 

 Pneumococci are widely distributed, and are almost universally 

 present in the mouth ; the necessary exciting cause, therefore, is 

 always at hand. Under ordinary circumstances the power of 

 resistance is sufficient to ward off the infection, but when this 

 barrier of immunity is broken down by certain adverse circum- 

 stances by excessive fatigue or starvation, by cold, or by an over- 

 dose of alcohol or other poison the pneumococcus gains access 

 to the tissues, and infection 1 occurs. The balanced contest 

 spoken of above then takes place. The pneumococcus grows in 

 the lungs and blood and produces a toxin, which tends to reduce 

 the general health and the resistance of the body still further ; and 

 looking at the problem only from this side, it would appear that 

 the process would go on until all the immunity was broken down, 

 and the pneumococcus could flourish unchecked. This, indeed, 

 might perhaps happen did not death supervene and bring with it 

 conditions unfavourable for the growth of this organism. But all 

 this time the tissues of the host have been reacting, and (in non- 

 fatal cases) sooner or later a condition is brought about in which 

 the noxious power of the coccus and the immunity of the patient 

 are exactly level, so that the disease neither advances nor retro- 

 cedes; and the process goes still farther, and the patient develops 

 such a degree of resistance as will not only render him immune to 

 the spread of the infection, but will suffice to sterilize his tissues 

 of the pneumococci which have already gained access. In other 

 words, there has been an acquisition of immunity; the patient has 

 become immune to the pneumococcus, and it is this, and this 

 only, which has brought about the cure of the disease. 



This process may be represented very diagrammatically, as 

 shown on p. 6. 



The line ag represents the degree of immunity to the organism 

 in question, the pneumococcus. At b some event takes place 

 (e.g., exposure to cold) by which the resistance is lowered to such 

 a degree that infection can occur. This takes place at c, with the 

 result that the immunity falls still farther. At this time the 

 bacteria begin to flourish in the tissues in increasing numbers. 

 This is represented by the ascending line i. The immunity falls 

 and bacterial action increases until a certain point is reached, 



1 I have elsewhere defined infection as the access of living, virulent, 

 pathogenic bacteria to a region whence their toxins may act on the tissues of 

 the body (Rose and Carless's " Surgery," sixth edition et seq., chap. i.). 



