CHAPTEK I 



THE BACILLUS PESTIS THE ESSENTIAL CAUSE OF 

 ORIENTAL PLAGUE 



The literature of Plague, from the earliest historical 

 periods, when the disease was recognised to be a com- 

 municable disease, down to 1894, i.e. down to the outbreak 

 of plague in Hong-Kong, contains a number of suggestions 

 and assumptions as to the causation of the disease. But 

 as is the case with other communicable diseases, before 

 the * discovery of the actual contagium no scientific 

 distinction was or could be made between the primary or 

 essential cause, i.e. the causa causans, and those secondary 

 conditions which contribute to, and which favour infection : 

 terrestrial influences, peculiar atmospheric states, social 

 defects, famine and want, crowded and ill - ventilated 

 habitations, decomposing corpses, decomposed, insufficient, 

 and unclean food-stuffs, and a number of other conditions 

 which are apt to weaken and to influence in an unfavour- 

 able sense the resistance of the individual ; that is to say, 

 all conditions which in most infectious diseases play a part 

 in facilitating and enhancing infection were formerly con- 

 sidered as being of the nature of essentials. The discovery 

 of the Bacillus pestis, however, as the true essence of the 



B 



