EPIDEMICS 



103 



Epidemics. — Many of the contagious diseases attach them- 

 selves more or less permanently to certain districts, and are 

 termed endemic. Thus leprosy is endemic in the Sandwich 

 Islands, the Cape, etc. ; cholera is endemic in the delta of 

 the Ganges ; small-pox in the Soudan, and to a lesser 

 extent such diseases as diphtheria, typhoid and scarlet 

 fever, are endemic in some parts of England. From time 

 to time some of the specific diseases, particularly cholera, 

 and plague, become widely spread over certain areas; they 

 are then said to be epidemic. When a disease spreads 

 more or less over the globe, as in the case of influenza, it 

 is said to be a pandemic. The causes contributing to 

 epidemics are still but Httle known. They have been 

 attributed to meteorological and climatic conditions, im- 

 perfect and filthy sanitary conditions, particularly in the 

 case of cholera, to facilities for convection, accumulation of 

 susceptible individuals, etc. There are at least three causes 

 contributing to the rise of an epidemic, of which only two 

 are at present known. The first factor, which we may call 

 X, is the specific micro-organism of the disease in question ; 

 the second factor, z, is individual idiosyncrasy, climatic and 

 other predisposing causes; but the third factor, y, the 

 cause of epidemics, still remains to be determined. This 

 is the X, y, z theory of Dr. Von Pettenkofer, and it is 



