﻿NO. 8 RECOVERY FROM INFECTION FLEXNER 7 



eases ; that is, diseases of occasional occurrence. This is true of such 

 epidemic diseases as influenza, meningitis, poliomyelitis, bubonic 

 plague, and many others. But when the particular conditions appear 

 that make possible the transfer of the adapted infectious germs 

 quickly from susceptible individual to susceptible individual, then 

 epidemics tend to arise. The period of this rapid transfer tends 

 to synchronize with certain seasons, so that temperature and other 

 meteorologic conditions play a part, affecting probably the parasites 

 less than the animal host. Thus epidemic meningitis prevails in the 

 winter, doubtless because the nasopharynx is less well defended 

 through the occurrence of colds, etc., than in warm weather. Ac- 

 cidental conditions attending seasons and climates may also play an 

 important role in particular epidemics. The recent Manchurian 

 epidemic of plague, which claimed in a few weeks no less than 45,000 

 victims, was started, doubtless, by infected marmots which were 

 being trapped for their skins ; but its spread and conversion into the 

 deadly pneumonic disease was promoted, if not directly caused, by 

 the rigors of the Manchurian winters and the primitive accommo- 

 dations in railways and inns for the thousands of coolie trappers who 

 were returning southward to their homes. It is a recent and grue- 

 some story how the desperate and panic-stricken coolies, in a fruitless 

 effort to escape death, carried far and wide the highly virulent 

 infection. 



At an earlier era, knowing nothing of microscopic life and the 

 essential part it plays in epidemics of disease, and recognizing that 

 these catastrophes were not entirely willful but arose and declined 

 according to some measure of law, physicians imagined a mystical 

 " genus epidemicus " which controlled the visitations of the scourge. 

 Modern science has indeed exorcised this ancient demon; and 

 although it has not yet determined with precision all the elements 

 that enter into the phenomenon of epidemics, it has unravelled them 

 in so far as to indicate that they relate not merely to parasites but 

 even more to the habits and customs, the beliefs and superstitions 

 of peoples, that are often quite as difficult and yet as necessary to alter 

 and improve as it is to overcome the malign propensities of the para- 

 sites themselves. 



I have indicated that the body possesses remarkable external de- 

 fensive mechanisms against infection, and, therefore, in order that 

 infection may be accomplished, these defenses must at some point be 

 weakened and destroyed ; and I have also sketched some of the means 

 employed to penetrate this armor. It may be illuminating if I now 



