352 POPULAR SCIENCE MONTHLY. 



The termination of an epidemic in the pre-sanitary period depended 

 to a considerable extent upon the fact that those who suffered a mild 

 attack acquired thereby an immunity; and that when the more sus- 

 ceptible individuals in a community had succumbed to the prevailing 

 disease there was a necessary termination of the epidemic for want of 

 material. 



Another factor which no doubt has an important bearing upon the 

 termination of epidemics is a change in the virulence of the germ as 

 a result of various natural agencies. Time will not permit me to dis- 

 cuss this subject in its scientific and practical aspects, but the general 

 fact may be stated that all known disease germs may vary greatly in 

 their pathogenic virulence, and that in every infectious disease mild 

 cases may occur, not only because of the slight susceptibility of the 

 individual, but also because of the 'attenuated' virulence of the specific 

 germ. In the eighteenth century, the beginning of sanitary science, 

 isolation of the sick and seaboard quarantines came to the aid of these 

 natural agencies, and did much in the way of arresting the progress of 

 this pestilential disease. At the present day these measures, together 

 with disinfection by heat or chemical agents, are relied upon by sani- 

 tarians with great confidence as being entirely adequate for the ex- 

 clusion of this disease or for stamping it out if it should effect a lodg- 

 ment in localities where an enlightened public sentiment permits the 

 thorough execution of these preventive measures; but when the disease 

 prevails among an ignorant population which strenuously objects to 

 the carrying out of these measures, the contest between the sanitary 

 officer and the deadly germ is an unequal one, and the stamping out of 

 an epidemic becomes a task of great magnitude, if not entirely hopeless. 

 This is illustrated by the experience of the English in their encounter 

 with bubonic plague in their Indian Empire. 



Plague seemed to be almost a thing of the past and no longer gave 

 any uneasiness in the countries of Europe which had formerly suffered 

 from its ravages, when in February, 1894, it made its appearance in 

 the city of Canton-) China, and three months later in Hong Kong. The 

 disease is known to have been epidemic in the province of Yunnan, 

 which is about 900 miles distant from Canton, since the year 1873, but 

 it attracted little attention until the lives of Europeans living in the 

 city of Hong Kong were threatened by the outbreak of an epidemic 

 among the Chinese residents of that place. Many thousands of deaths 

 occurred in Canton during the three months which elapsed after its 

 introduction to that city before it effected a lodgment in Hong Kong. 



Fortunately this outbreak gave the opportunity for competent bac- 

 teriologists to make scientific investigations relating to the specific 

 cause of this scourge of the human race and to the demonstration that 

 it is due to a minute bacillus. This discovery was first made by the 



