PHENOMENA OF INFECTION. 67 



ena which concern the body on the one hand, and the 

 infectious agents on the other. To illustrate: We say 

 that the pneumococcus is the cause of pneumonia 

 because this particular micro-organism is found in the 

 characteristic expectoration during life, and in the 

 lungs after death. Unquestionably, it is a necessary 

 factor in the disease ; but is it the sole factor ? We have 

 only to examine the facts in the case to discover that 

 it is not. In the first place, practically everyone, even 

 in health, harbors the pneumococcus in his mouth. 

 In the second place, pneumonia in robust individuals 

 invariably follows either fatigue and exposure to in- 

 clement weather, or exposure and over-indulgence in 

 intoxicants; often a combination of all three factors 

 precedes the onset. In the less robust, e.g., those 

 suffering from chronic ailments— (a condition in which 

 susceptibility to pneumonia is especially marked) it is 

 the pre-existing sub-normal state of the health which 

 permits the micro-organisms to invade the deeper air- 

 passages and produce the disease. Again: It is a mat- 

 ter of common observation that of a number of indi- 

 viduals exposed to the same infectious disease not all 

 are attacked, and that in those susceptible the disease 

 presents extraordinary variations as regards its mild- 

 ness or severity. If the microbe were the sole factor 

 concerned, all exposed individuals would be attacked, 

 and all attacked would suffer to the same degree; a 

 thing which everyone knows never happens. More- 



