414 APPLICATION OF METHODS OF BACTERIOLOGY 



trustworthy evidence is to the contrary, arid we now believe 

 that with pneumococci, as with all other living things, there 

 have been established in the course of time, as a result of 

 environmental influences, variations of a type species result- 

 ing in the acquisition of essentially fixed characters of 

 fundamental importance. 



The Mechanism of Pneumonic Infection The most impor- 

 tant result of pneumococcus infection in man is pneumonia. 

 The mechanism of the origin, course and recovery from 

 pneumonia still constitutes o"ne of the obscure problems 

 of medicine, even though special investigations have shed 

 much light upon several important phases of the subject. 



For a clear appreciation of the current views on the 

 essential features of this riddle, we must bear in mind several 

 fundamental facts: 



1. That pneumonia is not invariably the consequence of 

 the presence of pneumococci upon the mucous surfaces or 

 in the body, for that organism is often found, fully virulent 

 for experimental animals in the mouth, nose or upper air 

 passages of persons in perfect health. ("Carriers" and 

 "Contacts.") 



2. That pneumonia, when not terminating fatally, is a 

 self -limited disease, i. e., the signs and symptoms increase 

 from the start until a point is reached, "the crisis," when 

 their severity suddenly begins to lessen and may continue 

 to do so until recovery is established. 



3. That up to, and for a time after the crisis, often far 

 into convalescence, living virulent pneumococci are present 

 in the lungs. They can be found constantly in the sputum 

 and often in smaller or larger numbers in the circulating 

 blood. Their number seems at times to be affected little, 

 if at all, by the forces that occasion the crisis. 



